Some properties of the Kilbas-Saigo function

We characterize the complete monotonicity of the Kilbas-Saigo function on the negative half-line. We also provide the exact asymptotics at $-\infty$, and uniform hyperbolic bounds are derived. The same questions are addressed for the classical Le Roy function. The main ingredient for the proof is a probabilistic representation of these functions in terms of the stable subordinator.


Introduction
The Kilbas-Saigo function is a three-parameter entire function having the convergent series representation E α,m,l (z) = 1 + n≥1 n k=1 Γ(1 + α((k − 1)m + l)) Γ(1 + α((k − 1)m + l + 1)) z n , z ∈ C, where the parameters are such that for α, m > 0 and l > −1/α. It can be viewed as a generalization of the one-or two-parameter Mittag-Leffler function since, with the standard notation for the latter, for every α, β > 0 and z ∈ C. This function was introduced in [13] as the solution to some integro-differential equation with Abelian kernel on the half-line, and we refer to Chapter 5.2 in [10] for a more recent account, including an extension to complex values of the parameter l. In our previous paper [4], written in collaboration with P. Vallois, it was shown that certain Kilbas-Saigo functions are moment generating functions of Riemannian integrals of the stable subordinator.
This observation made it possible to define rigorously some Weibull and Fréchet distributions of fractional type via an independent exponential random variable and the stable subordinator -see [4] for details. In the present paper, we wish to take the other way round and use the probabilistic connection in order to deduce some non-trivial analytical properties of the Kilbas-Saigo function.
In Section 2, we tackle the problem of the complete monotonicity on the negative half-line.
This problem dates back to Pollard in 1948 for the one-parameter Mittag-Leffler function -see e.g. Section 3.7.2 in [10] for details and references. It was shown in [4] that for every m > 0 and α ∈ (0, 1] the function x → E α,m,m−1 (−x) is completely monotone, extending Pollard's result and solving an open problem stated in [6]. In Theorem 2.3 below, we characterize the complete monotonicity of x → E α,m,l (−x) by α ∈ [0, 1] and l ≥ m − 1/α. We also give an explicit representation, albeit complicated in general, of the underlying positive random variable. Along the way, we study an interesting family of Mellin transforms given as the quotient of four double Gamma functions.
In Section 3, we establish uniform hyperbolic bounds on the negative half-line for two families of completely monotonic Kilbas-Saigo functions, extending the bounds obtained in [22] for the classical Mittag-Leffler function. The argument in [22] relied on stochastic and convex orderings and was rather lenghty. We use here the same kind of arguments, but the proof is shorter and more transparent thanks to the connection with the stable subordinator. The latter also implies some monotonicity properties on m → E α,m,l (x) for every x ∈ R -see Proposition 3.1 below.
In Section 4, we address the question of the asymptotic behaviour at −∞ in the completely monotonic case α ∈ (0, 1] and l ≥ m − 1/α. It is shown in Theorem 5.5 of [10] that in the general case α, m > 0 and l > m − 1/α, the entire function E α,m,l (z) has order ρ = 1/α and type σ = 1/m. In the last Section 5, we pay attention to the so-called Le Roy function with parameter α > 0.
The latter is a simple generalization of the exponential function defined by Introduced in [16] in the context of analytic continuation, a couple of years before the Mittag-Leffler function, the Le Roy function has been much less studied. It was shown in [4]  It is worth mentioning that the latter property is an open problem -see Conjecture 5.6 below -for the Mittag-Leffler function.
As in [4], an important role is played throughout the paper by Barnes' double Gamma function G(z; δ) which is the unique solution to the functional equation G(z + 1; δ) = Γ(zδ −1 )G(z; δ) with normalization G(1; δ) = 1, and its associated Pochhammer type symbol We have gathered in an Appendix all the needed facts and formulae on this double Gamma function, whose connection with the Kilbas-Saigo function has probably a broader focus than the content of the present paper (we leave this topic open to further research).

Complete monotonicity on the negative half-line
In this section, we wish to characterize the property that the function x → E α,m,l (−x) is completely monotone (CM) on (0, ∞). We begin with the following result on the above generalized Pochhammer symbols, which is reminiscent of Proposition 5.1 and Theorem 6.2 in [7] and has an independent interest.
Proof. We giscard the degenerate case a = b = c = d, which is obvious with Z = 1. By (A.2) and some rearrangements -see also (2.15) in [17], we first rewrite for every s > 0, where κ is some real constant. By convexity, it is easy to see that if b + d ≤ a + c and inf{b, d} ≤ inf{a, c}, then the function z → z b + z d − z a − z c is positive on (0, 1). This implies that the function By the Lévy-Khintchine formula, this shows that the ID random variable Y is negative. Moreover, its support is (−∞, 0] since its Lévy measure has full support and its drift coefficient is zero -see Theorem 24.10 (iii) in [20], so that the support of Z is [0, 1].
Assuming second b + d < a + c, the same Taylor expansion as above shows that the density of the Lévy measure of Y does not integrate 1 ∧ |x| and the real Lévy process associated to Y is hence of type C with the terminology of [20] -see Definition 11.9 therein. By Theorem 24.10 (i) in [20], this implies that Y has full support on R, and so does Z on R + .
we obtain the decomposition whose right-hand side is the Laplace exponent of some ID random variable U having an atom because its Lévy measure, whose support is bounded away from zero, is finite -see Theorem over (0, ∞) for any s 0 > − inf{b, d}. From this expression, it is possible to prove that this density is real-analytic over the interior of the support. We omit details. Let us also mention by Remark 28.8 in [20] that this density is positive over the interior of its support.
(b) With the standard notation for the Pochhammer symbol, the aforementioned Proposition 5.1 and Theorem 6.2 in [7] show that dx.
This expression also shows that the underlying random variable has support [0, 1] and that it is absolutely continuous, save for a + c = b + d where it has an atom at zero. We refer to [7] for an exact expression of the density on (0, 1) in terms of the classical hypergeometric function.
We can now characterize the CM property for E α,m,l (−x) on (0, ∞).
for every z ≥ 0, where in the third equality we have used (A.1) repeatedly. The latter identity is extended analytically to the whole complex plane and we get, in particular, x ≥ 0.
This shows that E α,m,l (−x) is CM with the required Bernstein representation.
We now prove the only if part. If E α,m,l (−x) is CM, then we see by analytic continuation that E α,m,l (z) is the moment generating function on C of the underlying random variable X, whose positive integer moments read bounded on {ℜ(s) = 0}, and has at most exponential growth on {ℜ(s) > 0} because by Hölder's inequality. On the other hand, the Stirling type formula (A.4) implies, after some simplifications, (1)) as |s| → ∞ with | arg s| < π and this shows that the function on the left-hand side, which is analytic on {ℜ(s) ≥ 0}, has at most linear growth on {ℜ(s) = 0} and at most exponential growth on {ℜ(s) > 0}. Moreover, the above analysis clearly shows that for all n ≥ 0 and by Carlson's theorem -see e.g. Section 5.81 in [24], we must have for every s > 0, a contradiction since Lemma 2.1 shows that the right-hand side cannot be the Mellin transform of a positive random variable if c < 1. The case α < 1 and l + 1/α < m is analogous. It consists in identifying the bounded sequence as the values at non-negative integer points of the function where the purposeless constant κ can be evaluated from (A.4). On {ℜ(s) ≥ 0}, we see that this function has growth at most e π(1−α)|s|/2 and we can again apply Carlson's theorem. We leave the details to the interested reader.
denotes, here and throughout, a standard Beta random variable with parameters a, b > 0. We hence recover the Bernstein representation of the CM function Γ(β)E α,β (−x) which was discussed in Remark 3.3 (c) in [4]. Notice also the very simple expression for the Mellin transform Another simplification occurs when l + 1/α = km for some integer k ≥ 1. One finds In general, the law of the absolutely continuous random variable X α,m,l valued in [0, 1] seems to have a complicated expression.
(c) As seen during the proof, the random variable Y α,m,l defined by the Bersntein representation with δ = 1/αm, for every s > −1. By Fubini's theorem, this implies the following exact computation, which seems unnoticed in the literature on the Kilbas-Saigo function.
Notice that there is no such Mellin-Barnes representation for E α,m,l (z) in general.

Uniform hyperbolic bounds
In Theorem 4 of [22], the following uniform hyperbolic bounds are obtained for the classical Mittag-Leffler function: for every α ∈ [0, 1] and x ≥ 0. The constants in these inequalities are optimal because of the asymptotic behaviours See [18] and the references therein for some motivations on these hyperbolic bounds. In this section, Those peculiar functions are associated to the fractional Weibull and Fréchet distributions defined in [4]. Specifically, we will use the following representations as a moment generating function, obtained respectively in (3.1) and (3.4) therein: Observe that these two formulae specify the general Bernstein representation (2.2) in terms of the α−stable subordinator only. We begin with the following monotonicity properties, of independent interest.
Proof. This is a consequence of (3.2) resp. (3.3), and the fact that σ  and any l ≤ 1/α. In the case l ∈ {1, 1/α}, this would require from (2.2) a monotonicity analysis of the mapping m → X α,m,m−l , which does not seem easy at first sight.
As in [22], our analysis to obtain the uniform bounds will use some notions of stochastic ordering.
Another ingredient in the proof is the following infinite independent product We refer to Section 2.1 in [17] for more details on this infinite product, including the fact that it is a.s. convergent for every a, b, c > 0. We also mention from Proposition 2 in [17] that its Mellin transform is The following simple result on convex orderings for the above infinite independent products has an independent interest.
Proof. By the definition of T(a, b, c) and the stability of the convex order by mixtures -see Corollary for every a, b > 0 and c ≥ b. Using again Corollary 3.A.22 in [21] and the standard identity which is a consequence of Jensen's inequality.
The following result is a generalization of the inequalities (3.1), which deal with the case m = 1 only, to all Kilbas-Saigo functions E α,m,m−1 (−x). The argument is considerably simpler than in the original proof of (3.1).
Theorem 3.4. For every α ∈ [0, 1], m > 0 and x ≥ 0, one has Proof. The first inequality is a consequence of Proposition 3.1, which implies in letting m → 0 where the first equality follows from Theorem 1.2 (b) (ii) in [17]. For the second inequality, we come back to the infinite product representation which follows from Theorem 1.2 (b) (i) in [17], exactly as in the proof of Theorem 1.1 in [4]. Lemma

implies then
where the identity in law follows from (2.7) in [17]. Using (3.2) with ρ = αm and the convexity of t → e −xt , we obtain the required As for the classical case m = 1, these bounds are optimal because of the asymptotic behaviours The behaviour at zero is plain from the definition, whereas the behaviour at infinity will be given after Remark 4.2 below.
(b) It is easy to check that the above proof also yields the upper bound for every α ∈ [0, 1], m > 0 and x ≥ 0, which seems unnoticed even in the classical case m = 1.
Our next result is a uniform hyperbolic upper bound for the Kilbas-Saigo function E α,m,m− 1 α (−x), with a power exponent which will be shown to be optimal in Remark 4.7 (c) below, and also an optimal constant because Proposition 3.6. For every α ∈ (0, 1], m > 0 and x ≥ 0, one has The inequality is derived by convex ordering as in Theorem 3.4: setting, here and throughout, Γ a for a Gamma random variable with parameter a > 0, one has where the first identity follows from Corollary 3 in [17] as in the proof of Theorem 1.1 in [4], the convex ordering from Lemma 3.3 and the second identity from (2.7) in [17]. Then, using (3.3) with ρ = αm, we get the required inequality.
As in Theorem 3.4, we believe that there is also a uniform lower bound, with a more complicated optimal constant which can be read off from the asymptotic behaviour of the density at zero obtained in Proposition 4.6 below: Conjecture 3.7. For every α ∈ (0, 1], m > 0 and x ≥ 0, one has Unfortunately, the proof of this general inequality still eludes us. The monotonicity property observed in Proposition 3.1 does not help here, giving only the trivial lower bound zero. The discrete factorizations which are used in [22] are also more difficult to handle in this context, because the Mellin transform underlying E α,m,m− 1 α is expressed in terms of generalized Pochhammer symbols. In the case m = 1, we could however get a proof of (3.4). The argument relies on the following representation, observed in Remarks 3.1 (d) and 3.3 (c) of [4]: is the first-passage time above one of the α−stable subordinator and T α its usual size-bias of order one.
Proposition 3.8. For every α ∈ (0, 1) and x ≥ 0, one has Proof. By (3.5) and since for every x ≥ 0, it is enough to show, reasoning exactly as in the proof of Theorem 4 in [22], that where ≺ st stands for the usual stochastic order between two real random variables. Recall that for every x ∈ R. Since T 1/2 d = 2 Γ 1/2 , the case α = 1/2 is explicit and the stochastic ordering can be obtained directly. More precisely, the densities of both random variables in (3.6) are respectively given by The argument for the case α = 1/2 is somehow analogous, but the details are more elaborate because the density of T α is not explicit anymore. We proceed as in Theorem C of [22] and first consider the case where α is rational. Setting α = p/n with n > p positive integers and X α = T (1) α we have, on the one hand, for every s > −2n −1 , with the notation for every s > −3n −1 , by factorization and Theorem 1.A.3(d) in [22] we are finally reduced to show for every n > p positive integers. The latter is equivalent to and this is proved via the single intersection property exactly as for (5.1) in [22]: the random variable on the left-hand side has an increasing density on (0, 1), whereas the random variable on the right-hand side has a decreasing density on (0, ∞), both densities having the same positive finite value at zero. We omit details. This completes the proof of (3.6) when α is rational. The case when α is irrational follows then by a density argument.
Remark 3.9. It is easy to check from (A.5) and (A.6) that Our last result in this section gives optimal uniform hyperbolic bounds for the generalized Mittag-Leffler functions E α,β (−x) whenever they are completely monotone, that is for β ≥ α -see the above Remark 2.4 (a). This can be viewed as another generalization of (3.1).
Proposition 3.10. For every α ∈ (0, 1], β > α and x ≥ 0, one has 1 Proof. The bounds for E α,α (−x) are a direct consequence of (3.5), Proposition 3.6 and Proposition 3.8. Notice that letting α → 1 leads to the trivial bound 0 ≤ e −x ≤ (2/(2 + x)) 2 . To handle the bounds for β > α, we first recall from Remark 2.4 (a) that for every s > −1, which implies the factorization L d = Y α,1,l × (Γ β ) α . Since, by Jensen's inequality, we deduce from Corollary 3.A.22 in [21] the convex ordering The argument for the other inequality is analogous to that of Proposition 3.8. By density, we only need to consider the case α = p/n and β = (p + q)/n with p < n and q positive integers. By Comparing these two formulas, we are reduced to show for every p < n and q positive integers. This is obtained in the same way as above via the single intersection property. We leave the details to the reader.

Asymptotic behaviour of fractional extreme densities
In this section which is a complement to [4], we study the behaviour of the density functions of whereF = 1− F denotes the associated survival function and D α 0+ a progressive Liouville fractional derivative on (0, ∞). The case α = 1 corresponds to the standard Weibull distribution. In [4], it is shown that this distribution function exists and is given by for every x ≥ 0 -see the formula following (3.1) in [4]. In particular, the density f W α,λ,ρ is real-analytic on (0, ∞) and has the following asymptotic behaviour at zero: The behaviour of f W α,λ,ρ at infinity is however less immediate, and to this aim we will need an exact expression for the Mellin transform of the random variable W α,λ,ρ with distribution function F W α,λ,ρ , which has an interest in itself.
for every s ∈ (−ρ, ρ). As a consequence, one has Proof. We start with a more concise expression of (2.3) for l = m − 1, which is a direct consequence of (A.9): By Theorem 1.1 in [4] and using the notations therein, we deduce for every s ∈ (−ρ, ρ) as required, where the third equality comes from (A.8). The asymptotic behaviour of the density at infinity is then a standard consequence of Mellin inversion. First, we observe from the above formula and (A.10) that the first positive pole of s → E W s α,λ,ρ is simple and isolated in the complex plane at s = ρ, with as s ↑ ρ, where the second asymptotics comes from (A.9) and the equality from (A.5). Therefore, applying Theorem 4 (ii) in [8] -beware the correction (log x) k → (log x) k−1 to be made in the expansion of f (x) therein, we obtain x −ρ−1 as x → ∞ as required. in [4] and the notation therein, we see by multiplicative convolution, having set f G α,ρ for the density of the generalized stable random variable G(ρ where for the asymptotics we have used the Proposition in [12] and a direct integration.
This argument does not make use of Mellin inversion and is overall simpler than the above. However, it does not convey to the fractional Fréchet case.
in accordance with Remark 3.1 (d) in [4]. This yields an identity which was already discussed for λ = 1 in the introduction of [4] as the solution (c) The two cases ρ = α and ρ = 1 − α have a Mellin transform expressed as the quotient of a finite number of Gamma functions. This makes it possible to use a Mellin-Barnes representation of the density in order to get its full asymptotic expansion at infinity. Using the standard notation of Definition C.1.1 in [1], one obtains which are everywhere divergent. The first expansion can also be obtained from (1.8.28) in [14] using f W α,λ,α (x) = λ x α−1 E α,α (−λx α ). Unfortunately, the Mellin transform of W α,λ,ρ might have poles of variable order and it seems difficult to obtain a general formula for the full asymptotic expansion at infinity of f W α,λ,ρ (x). Writing f W α,λ,ρ (y) dy, we obtain by integration the following asymptotic behaviour at infinity, which is valid for any α ∈ (0, 1] and m > 0 : This behaviour, which turns out to be the same as that of the classical Mittag-Leffler function Proof. The case α = 0 is obvious since E 0,m,l (x) = 1/(1 − x). For α ∈ (0, 1], setting δ = 1/αm, recall from (2.4) that for every s ∈ (0, 1) one has  We end this paragraph with the following conjecture which is natural in view of Proposition 4.3.
We know by Theorem 3.4 resp. Proposition 3.10 that this conjecture is true for the cases l = m − 1 and m = 1. and λ, ρ > 0 is defined as the unique distribution function F F α,λ,ρ on (0, ∞) solving the fractional differential equation where D α − denotes a regressive Liouville fractional derivative on (0, ∞). The case α = 1 corresponds to the standard Fréchet distribution. In [4], it is shown that this distribution function exists and is given by for every x ≥ 0 -see the formula following (3.4) in [4]. In particular, the density f F α,λ,ρ is real-analytic on (0, ∞) and has the following asymptotic behaviour at infinity: The behaviour of the density at zero is less immediate and we will need, as in the above paragraph, the exact expression of the Mellin transform of the random variable F α,λ,ρ with distribution function F F α,λ,ρ , whose strip of analyticity is larger than that of W α,λ,ρ .
Proposition 4.6. The Mellin transform of F α,λ,ρ is for every s ∈ (−ρ − α, ρ). As a consequence, one has Proof. The evaluation of the Mellin transform is done as for the fractional Weibull distribution, starting from the expression which is a consequence of (2.3) and (A.5). By Theorem 1.2 in [4] and (A.8), we obtain the required The asymptotic behaviour of f F α,λ,ρ (x) at zero follows then as for that of f W α,λ,ρ (x) at infinity, in considering the residue at the first negative pole s = −(ρ + α) which is simple and isolated in the complex plane, applying Theorem 4 (i) in [8] -with the same correction as above, and making various simplifications. We omit details.
in accordance with the scaling property F α,λ,ρ d = λ 1/ρ F α,1,ρ and the identities given after the statement of Theorem 1.2 in [4]. The Mellin transform also takes a simpler form in the same other situations as above.
• For ρ = α, with This yields the identity F α,λ,α , which was discussed for λ = 1 in the introduction of [4] as the solution to (1.4) therein. This is also in accordance with Remark 3.3 (c) in [4], since Notice that the constant appearing in the asymptotic behaviour of the density at zero is also simpler: one finds Here, the density converges at zero to a simple constant: one finds (c) Integrating the density and using P[F α,λ,ρ ≤ x] = E α, ρ α , ρ−1 α (−λx −ρ ), we obtain the following asymptotic behaviour at infinity for any α ∈ (0, 1] and m > 0, which is more involved than that of Proposition 4.3: For m = 1, this behaviour matches the first term in the full asymptotic expansion As for E α,m,m−1 (−x), a full asymptotic expansion of E α,m,m− 1 α (−x) at infinity seems difficult to obtain for all values of m.

Some complements on the Le Roy function
In this section we show some miscellaneous results on the Le Roy function In [4], this function played a role in the construction of a fractional Gumbel distribution -see Theorem 1.3 therein. The Le Roy function, which has been much less studied than the classical Mittag-Leffler function, can be viewed as an alternative generalization of the exponential function.
Throughout, we giscard the explicit case L 1 (x) = E 1 (x) = e x .
We begin with the asymptotic behaviour at infinity. Le Roy's original result -see [16] p. 263 - and is obtained by a variation on Laplace's method. An extension of this asymptotic behaviour was recently given in [9] for the so-called Mittag-Leffler functions of Le Roy type. Laplace's method can also be used to solve Exercise 8.8.4 in [19], which states for α ≥ 2 and for α ∈ (1, 2), as x → ∞. The following estimate, which seems to have passed unnoticed in the literature, completes the picture.
Proposition 5.1. For every α ∈ (0, 1), one has Proof. In the proof of Theorem 1.3 in [4] it is shown that dt has density f α on (0, ∞) and Mellin transform In particular, we have f α = e 1−α with the notation of [2] and Theorem 2.4 therein implies Plugging this estimate into the above expression for L α (−x), we conclude the proof by a direct integration.
Remark 5.2. (a) The estimate (5.3) also gives the asymptotic behaviour, at the right end of the support, of the density of the fractional Gumbel random variable G α,λ which is defined in Theorem 1.3 of [4]. Indeed, by the definition and multiplicative convolution the density of e λG α,λ on (0, ∞) where the estimate follows from (5.3) as in the proof of Proposition 5.1. A change of variable implies then Notice that at the left end of the support, there is a convergent series representation which is given by Corollary 3.6 in [4].
(b) In the case α = 2, one has L 2 (x) = I 0 (2 √ x) and L 2 (−x) = J 0 (2 √ x) for all x ≥ 0, where I 0 and J 0 are the classical, modified or not, Bessel functions with index 0. In particular, a full asymptotic expansion for L 2 at both ends of the support is available, to be deduced e.g. from Our next result characterizes the connection between the entire function L α (z) and random variables. Recall that a function f : C → C which is holomorphic in a neighbourhood Ω of the origin is a moment generating function (MGF) if there exists a real random variable X such that f (z) = E e zX , z ∈ Ω.
In particular, it is clear that L 0 is the MGF of the exponential law L and L 1 that of the constant variable 1. The following provides a characterization. In this case, one has Proof. The if part is a direct consequence of the proof of Proposition 5.1. On the other hand, the estimates (5.1) and (5.2) show that L α (z) takes negative values on R − , so that it cannot be the moment generating function of a real random variable, when α > 1. This completes the proof.
Observe that since L α is non-negative, the above result also shows L α (−x) is CM on (0, ∞) if and only if α ≤ 1, echoing Pollard's aforementioned classical result for the Mittag-Leffler E α (−x).
One can ask whether there are further complete monotonicity properties for L α , as in [23] for E α .
Our last result for the Le Roy function is a monotonicity property which is akin to Proposition 3.1.
Proof. The fact that α → L α (x) decreases on R + is obvious for x ≥ 0, by the definition of L α .
To show the property on [0, 1] for x < 0, we will use a convex ordering argument. More precisely, the Malmsten formula (A.3) and the Lévy-Khintchine formula show that for every t ∈ [0, 1], the random variable G 1−t = log L 1−t is the marginal at time t of a real Lévy process, since E[e izG 1−t ] = Γ(1 + iz) t = e tψ(z) for every z ∈ R, with This is actually well-known -see Example E in [5]. By independence and stationarity of the increments of a Lévy process, we deduce that there exists a multiplicative martingale {M t , t ∈ [0, 1]} such that M t d = L 1−t for every t ∈ [0, 1] and Jensen's inequality implies L β ≺ cx L α for every 0 ≤ α ≤ β ≤ 1. Applying the definition of convex ordering to the function ϕ(x) = e x , we get L β (x) ≤ L α (x) for every x < 0 and 0 ≤ α ≤ β ≤ 1, as required.
Remark 5.5. (a) In the terminology of [11], the family {L 1−α , α ∈ [0, 1]} is a peacock, whose associated multiplicative martingale is completely explicit. We refer to [11] for numerous examples of explicit peacocks related to exponential functionals of Lévy processes. Observe from Lemma 3.3 that the family {T(a, b, t), t > 0} is also a peacock.
(b) Letting α → 0 and α → 1 in Proposition 5.4 leads to the bounds e x ≤ L β (x) ≤ L α (x) ≤ 1 (1 − x) + for every x ∈ R and 0 < α < β < 1. The hyperbolic upper bound is optimal as in Propositions 3.4 and 3.6, because L α (x) − 1 ∼ x as x → 0. The exponential lower bound is thinner than the order given in Proposition 5.1. On the other hand, it does not seem that stochastic ordering arguments can help for a uniform estimate involving a logarithmic term.
It is natural to ask if the statement of Proposition 5.4 is also true for the classical Mittag-Leffler function, and this problem seems still open.
Numerical simulations suggest a positive answer. For x < 0, see the GIF animation given on the top of the english Wikipedia page of the Mittag-Leffler function [25]. It is clear by the definition that α → E α (x) is non-increasing for every x ≥ 0 on [α 0 , ∞), where 1 + α 0 = 1.46163... is the location of the minimum of the Gamma function on (0, ∞). A direct consequence of Theorem B in [22] is also that is non-increasing on [1/2, 1] for every x ∈ R. The constant Γ(1 + α) appears above because of the convex ordering argument used in [22]. It seems that other kinds of arguments are necessary to study the monotonicity of α → E α (x) on [0, 1].
We would like to finish this paper with the following related monotonicity result, which relies on a stochastic ordering argument, for the generalized Mittag-Leffler function.