The theory of differential equations with fractional derivatives has gained significant popularity and importance in the last few decades, mainly due to its applications in many seemingly distant fields of science and technology (see, for example, [1] - [6] ).
One of the most important time-fractional equations is the subdiffusion equation, which models anomalous or slow diffusion processes. This equation is a partial integro-differential equation obtained from the classical heat equation by replacing the first-order derivative with a time-fractional derivative of the order ρ ∈ (0, 1).
When considering the subdiffusion equation as a model equation in the analysis of anomalous diffusion processes, the order of the fractional derivative is often unknown and difficult to measure directly. To determine this parameter, it is necessary to investigate the inverse problems of identifying these physical quantities based on some indirectly observable information about solutions (see a survey paper Li, Liu and Yamamoto [7] ).
In this paper, we investigate the existence and uniqueness of solutions to initial-boundary value problems for subdiffusion equations with the Caputo derivative and the elliptic operator A(D) in R N with constant coefficients. The inverse problems of determining the order of the fractional derivative with respect to time and with respect to the spatial variable will also be investigated.
Let us proceed to a rigorous formulation of the main results of this article. h(ξ) (t − ξ) ρ+1 dξ, t > 0, provided the right-hand side exists. Here Γ(ρ) is Euler's gamma function. Using this definition one can define the Caputo fractional derivative of order ρ, 0 < ρ < 1, as
Let ρ ∈ (0, 1) be a given number. Consider the initial-boundary value problem
where ϕ(x) is a given continuous function. We call problem (1.1) -(1.3) the forward problem. Denoting the Sobolev classes by L τ 2 (R N ) (see the definition in the next section), we can state an existence theorem for this problem.
Then the forward problem has a solution in the form
The integral uniformly converges with respect to x ∈ R N and for each t ∈ (0, T ], whereφ(ξ) is the Fourier transform of ϕ.
If the solution of the forward problem u(x, t) ∈ L2(R N ), t ∈ (0, T ], then we may define the Fourier transformû
The corresponding uniqueness theorem has the form. Theorem 1.3. Let the following conditions be satisfied for all t ∈ (0, T ]
û(ξ, t) ∈ L1(R N ). Then there can be only one solution to the forward problem. Remark 1.4. We will prove that under the condition of Theorem 1.2 for initial function ϕ, all four conditions of Theorem 1.3 are also satisfied. Thus, if we add these four conditions to Definition 1.1, then Theorem 1.2 guarantees both the existence and the uniqueness of such a solution.
In recent years, numerous works of specialists have appeared, where they study various initialboundary value problems for various subdiffusion equations. Let us mention only some of these works. Basically, the case of one spatial variable x ∈ R and subdiffusion equation with "the elliptical part" uxx were considered (see, for example, handbook Machado, aditor [1] , book of A.A. Kilbas et al. [3] and monograph of A. V. Pskhu [8] , and references in these works). The paper Gorenflo, Luchko and Yamamoto [9] is devoted to the study of subdiffusion equations in Sobelev spaces. In the paper by Kubica and Yamamoto [10] , initial-boundary value problems for equations with time-dependent coefficients are considered. In multidimensional case (x ∈ R N ) instead of the differential expression uxx authors considered either the Laplace operator ( [3] , [11] - [13] ), or pseudo-differential operators with constant coefficients in the whole space R N (Umarov [14] ). In the last paper the initial function ϕ ∈ Lp(R N ) is such, that the Fourier transformφ is compactly supported. The authors of the recent paper [15] considered initialboundary value problems for subdiffusion equations with arbitrary elliptic differential operators in bounded domains.
2. Determining the correct order of an equation in applied fractional modeling plays an important role. The corresponding inverse problem for subdiffusion equations has been considered by a number of authors (see a survey paper Li, Liu and Yamamoto [7] and references therein, [16] - [21] ). Note that in all known works the subdiffusion equation was considered in a bounded domain Ω ⊂ R N . In addition, it should be noted that in publications [16] - [19] the following relation was taken as an additional condition
at a monitoring point x0 ∈ Ω. But this condition, as a rule (an exception is work [19] by J. Janno, where both the uniqueness and the existence are proved), can ensure only the uniqueness of the solution of the inverse problem [16] - [18] . Authors of paper Ashurov and Umarov [20] considered as an additional information the value of projection of the solution onto the first eigenfunction of the elliptic part of subdiffusion equation. Note, results of paper [20] are applicable only in case, when the first eigenvalue is equal to zero. The uniqueness and existence of an unknown order of the fractional derivative in the subdiffusion equation were proved in the recent work of Alimov and Ashurov [21] . In this case, the additional condition is ||u(x, t0)|| 2 = d0, and the boundary condition is not necessarily homogeneous. Now let us consider the order of fractional derivative ρ in equation (1.1) as an unknown parameter. We formulate our inverse problem in the following way. Let us fix a vector ξ0 = 0, such thatφ(ξ0) = 0 and put λ0 = A(ξ0) > 0. To determine the order ρ of the fractional derivative in (1.1) we use the following extra data: Let us denote by Eρ(t) the Mittag-Leffler function of the form
.
To solve the inverse problem fix the number ρ0 ∈ (0, 1) and consider the problem for ρ ∈ [ρ0, 1). Lemma 1.6. For ρ0 from the interval 0 < ρ0 < 1, there is a number T0 = T0(λ0, ρ0) such that for all t0 ≥ T0 and for arbitrary ϕ ∈ L2(R N ) the function U (t0, ρ) decreases monotonically with respect to ρ ∈ [ρ0, 1].
Remark 1.7. The number T0(λ0, ρ0) can be chosen as
where B1 = 43 and B2 = 4.6. But if ρ0λ0 > 0.0075, then you can just put T0 = 2.
The result related to the inverse problem has the form. 
3. Finally, we will consider another inverse problem of determining both the orders of fractional derivatives with respect to time and the spatial derivatives in the subdiffusion equations.
For the best of our knowledge, only in the following two papers [22] and [23] such inverse problems were studied and only the uniqueness theorems ware proved (note that uniqueness is a very important property of a solution from an application point of view). In paper [22] by Tatar and Ulusoy it is considered the initial-boundary value problem for differential equation
where △ σ is the one-dimensional fractional Laplace operator, ρ ∈ (0, 1) and σ ∈ (1/4, 1). The authors have proved that if the initial function ϕ(x) is sufficiently smooth and all its Fourier coefficients are positive, then the two-parameter inverse problem with additional information (1.5) has a unique solution. As for physical backgrounds for two-parameter differential equations, see, for example, [24] .
In [23] , M. Yamamoto proved the uniqueness theorem for the above two-parameter inverse problem in N -dimensional bounded domain Ω with smooth boundary ∂Ω. The conditions on the initial function found in this work are less restrictive, for example, if ϕ is zero on ∂Ω, ϕ ∈ L τ 2 (Ω), τ > N/2, ϕ ≥ 0 in Ω and ϕ(x0) = 0, then the uniqueness theorem is true.
It is easy to verify that the closureÂ of operator A is positive and selfadjoint. Therefore, by virtue of the von Neumann theorem, for any σ > 0, we can introduce the degree of the operatorÂ aŝ
where projectors P λ defined as
The domain of definition of this operator is determined from the conditionÂ σ f (x) ∈ L2(R N ) and has the form
Suppose first that ρ ∈ (ρ0, 1) and σ ∈ (0, 1) are given numbers and consider the initialboundary value (the second forward ) problem
where ϕ(x) is a given continuous function. The solution to this problem is defined similarly to the solution to problem (1.1) -(1.3) (see Definition 1.1). In exactly the same way as Theorem 1.2, it is proved that if a ϕ ∈ L τ 2 (R N ) and τ > N 2 , then the solution of the second forward problem has the form
where the integral uniformly converges in x ∈ R N and for each t ∈ (0, T ]. Now suppose, that parameters ρ and σ are unknown. To find these numbers one obviously needs two extra conditions. It should be noted, that the proposed method, for simultaneously finding both the order of fractional differentiation ρ and the power σ is applicable if there exists such ξ0 ∈ R N , so that A(ξ0) = 1 andφ(ξ0) = 0. Let ξ0 be one of such a vector. We consider the following information as additional conditions: (1.12) V (ξ0, t0, ρ, σ) = |v(ξ0, t0)| = d0, t0 ≥ T0(1, ρ0),
where ξ1 is such thatφ(ξ1) = 0 and Λ1 is defined in (4.2).
We call problem (1.8) -(1.10) together with extra conditions (1.12) and (1.13) the second inverse problem.
Note that V (ξ0, t0, ρ, σ) is actually independent of σ. Therefore, to solve the second inverse problem, we first find the unique ρ ⋆ that satisfies the relation (1.12). Then, assuming that ρ ⋆ is already known, using relation (1.13), we find the second unknown parameter σ ⋆ . Theorem 1.9. Let ϕ ∈ L τ 2 (R N ) and τ > N 2 . Then there exists unique ρ ⋆ , satisfying (1.12), if and only if d0 satisfies inequalities (1.7) with λ0 = 1. For σ ⋆ to exist, it is necessary and sufficient that d1 satisfies the inequalities
It should also be noted that the theory and applications of various inverse problems, on determining the coefficients of the equation, the right-hand side, and also on determining the initial or boundary functions for differential equations of integer order are discussed in Kabanikhin [25] (see also references therein).
In the present section we prove Theorems 1.2 and 1.3 and equation (1.11). The class of functions L2(R N ) which for a given fixed number a > 0 make the norm
finite is termed the Sobolev class L a 2 (R N ). Since for τ > 0 and some constants c1 and c2 one has the inequality
then D(Â τ ) = L τ m 2 (R N ). Let I be the identity operator in L2(R N ). Operator (Â + I) τ is defined in the same way as operatorÂ σ .
Proof of Theorem 1.2 is based on the following lemma (see M.A. Krasnoselski et al. [26] , p. 453), which is a simple consequence of the Sobolev embedding theorem.
Lemma 2.1. Let ν > 1 + N 2m . Then for any |α| ≤ m operator D α (Â + I) −ν continuously maps from L2(R N ) into C(R N ) and moreover the following estimate holds true
Proof. For any a > N/2 one has the Sobolev embedding theorem:
. Therefore, it is sufficient to prove the inequality
To prove the existence of the forward problem's solution we remind the following estimate of the Mittag-Leffler function with a negative argument (see, for example, [6] , p.29)
In accordance with Definition 1.1, we will first show that for function (1.4) one has A(D)u(x, t) ∈ C(R N × (0, T ]) (that is one can validly apply the operators D α , |α| ≤ m, to the series in (1.4) term-by-term).
Let τ > N 2 and ν = 1 + τ m > 1 + N 2m . Then
Therefore by virtue of Lemma 2.1 one has
Using the Parseval equality, we will have
This implies the uniform in x ∈ R N convergence of the differentiated sum (2.4) in the variables xj for each t ∈ (0, T ]. Further, from equation (1.1) one has D ρ t Sµ(x, t) = −A(D)Sµ(x, t). Therefore, proceeding the above reasoning, we arrive at D ρ t u(x, t) ∈ C(R N × (0.T ]). It is not difficult to verify that equation (1.1) and the initial condition (1.3) are satisfied (see, for example, [6] , page 173 and [27] ).
Let us show that the inclusion ϕ ∈ L τ 2 (R N ), τ > N/2, impliesφ ∈ L1(R N ). Indeed, 
The property of function ϕ :φ ∈ L1(R N ), established above, implies condition (4):
As regards condition (1) of Theorem 1.3, it is a direct consequence of the Sobolev embedding theorem and the condition ϕ ∈ L τ 2 (R N ), τ > N 2 , of Theorem 1.2.
Proof of Theorem 1.3. Let conditions (1)-(4) of Theorem 1.3 are satisfied. Observe, as it was shown above, Theorem 1.2 guarantee the fulfilment of these conditions. Suppose that problem (1.1)-(1.3) has two solutions u1(x, t) and u2(x, t). Our aim is to prove that u(x, t) = u1(x, t) − u2(x, t) ≡ 0. Since the problem is linear, then we have the following homogenous problem for u(x, t):
Let u(x, t) be a solution of problem (2.5)-(2.7). Since u(x, t) ∈ L2(R N ), t ∈ (0, T ] (see condition (3) of Theorem 1.3), we may define the Fourier transformû(ξ, t) and according to condition (4) one hasû(ξ, t) ∈ L1(R N ). Therefore, by virtue of Fubini's theorem, the following function of t exists for almost all λ:
where dσ λ (ξ) is the corresponding surface element.
Since u(x, t) is a solution of equation (2.5), then (note,
The inner integral exists as the Fourier transform of L2-function. In this integral, we integrate by parts. We will take into account the following: A(D) is a homogeneous symmetric and even order differential expression; conditions (2) of Theorem 1.3; and (2.6). Then
Therefore, we have the following Cauchy problem for w λ (t):
D ρ t w λ (t) + λw λ (t) = 0, t > 0; w λ (0) = 0. This problem has the unique solution; hence, the function defined by (2.8), is identically zero (see, for example, [6] , p. 173 and [27] ): w λ (t) ≡ 0 for almost all λ > 0. Integrating the equation (2.8) with respect to λ over the domain (0, +∞) we obtain R Nû (ξ, t)dξ = 0, t > 0. Therefore,û(ξ, t) = 0 for almost all ξ, or u(x, t) = 0 for almost all x and since u(x, t) continuous on x, then u(x, t) = 0 for all x and t. Thus Theorem 1.3 is proved.
Formula (1.11) for the solution of the second forward problem is established in exactly the same way with formula (1.4).
Lemma 3.1. Given ρ0 from the interval 0 < ρ0 < 1, there exists a number T0 = T0(λ0, ρ0), such that for all t0 ≥ T0 and λ ≥ λ0 function e λ (ρ) = Eρ(−λt ρ 0 ) is positive and monotonically decreasing with respect to ρ ∈ [ρ0, 1) and e λ (1) < e λ (ρ) ≤ e λ (ρ0).
Proof. Let us denote by δ(1; β) a contour oriented by non-decreasing arg ζ consisting of the following parts: the ray arg ζ = −β with |ζ| ≥ 1, the arc −β ≤ arg ζ ≤ β, |ζ| = 1, and the ray arg ζ = β, |ζ| ≥ 1. If 0 < β < π, then the contour δ(1; β) divides the complex ζ-plane into two unbounded parts, namely G (−) (1; β) to the left of δ(1; β) by orientation, and G (+) (1; β) to the right of it. The contour δ(1; β) is called the Hankel path.
Let β = 3π 4 ρ, ρ ∈ [ρ0, 1). Then by the definition of this contour δ(1; β), we arrive at (note, −λt ρ 0 ∈ G (−) (1; β), see [6] , p. 27)
Let Ψ(ρ) be the logarithmic derivative of the gamma function Γ(ρ) (for the definition and properties of Ψ see [28] ). Then Γ ′ (ρ) = Γ(ρ)Ψ(ρ), and therefore,
.
the function f ′ 1 (ρ) can be represented as follows
If γ ≈ 0, 57722 is the Euler-Mascheroni constant, then −γ < Ψ(2 − ρ) < 1 − γ. By virtue of this estimate we may write
On the other hand one has
Note, since β = 3π 4 ρ and ρ0 ≤ ρ < 1, then for a negative number z < 0 the following inequality holds min
Therefore,
where I = 2πρ(λt ρ 0 ) 2 . Let us denote the last three integrals by Jj, j = 1, 2, 3, correspondingly. Therefore,
It is not hard to verify, that
Consider the integral J1. Due to the presence of ln |ζ|, the integrand J1 is equal to 0 for |ζ| = 1. Moreover, on the rays arg ζ = ±β, β = 3π 4 ρ, one has
Hence (note ln |ζ| 
The integrands in J2 and J3 do not vanish on the sphere {|ζ| = 1} and the measure of the corresponding arc −β ≤ arg ζ ≤ β, |ζ| = 1, is equal to 2β. Therefore, using the same technique as above, we obtain
where B1 = 43 and B2 = 4.6. Therefore, taking into account estimate (3.2), we have
In other words, this derivative is negative if 1) or, which is the same,
Next, consider two cases: B1/ρ0 > B2 ln t0 and B1/ρ0 ≤ B2 ln t0. Recall that to satisfy inequality (3.2) we assumed that t0 ≥ 2.
Case 1. Let B1/ρ0 > B2 ln t0. Then
(the latter inequality is satisfied, say if t0 ≤ e B 1 /B 2 ). Therefore, inequality (3.4) is satisfied if
Thus, from the last two inequalities it follows that if If ρ0 and λ1 are such small numbers, that the first inequality of (3.5) does not hold true, then consider Case 2. In this case, t0 should be chosen large enough.
Thus, in Case 2 in order for the derivative (3.3) to be negative for all λ ≥ λ0 and ρ ∈ [ρ0, 1), it is sufficient that the following inequality takes place t0 ≥ T0, where (see Remark 1.7)
Finally, by virtue of inequality e λ (1) = e −λt > 0, one has e λ (ρ) > 0.
Since
Lemma 1.6 follows immediately from Lemma 3.1. Theorem 1.8 is an easy consequence of these two lemmas.
In conclusion, we make the following remark. If the elliptic polynomial A(ξ) is nonhomogeneous, that is A(ξ) = |α|≤m aαξ α and moreover, A(ξ) ≥ λ0 > 0, then from Lemma 3.1 it follows:
If t0 ≥ T0 and T0 is as above, then Eρ(−A(ξ)t ρ ), as a function of ρ, is positive and decreases monotonically in ρ ∈ [ρ0, 1] for any ξ ∈ R N . Therefore, in this case you can also consider various options for the function U (t, ρ). Examples U (t, ρ) = ||Au(x, t)|| 2 , U (t, ρ) = ||u(x, t)|| 2 , U (t, ρ) = (u, ϕ).
To prove Theorem 1.9, we first find the unknown parameter ρ. Suppose, as required by Theorem 1.9, that d0 satisfies condition (1.7) with λ0 = A(ξ0) = 1. Then, as it follows from Lemma 3.1, for all t0 ≥ T0(1, ρ0) the equation V (ξ0, t0, ρ, σ) = |v(ξ0, t0)| = Eρ(−t ρ )|ϕ(ξ0)| = d0 has the unique solution ρ ⋆ ∈ (ρ0, 1). Now let us define σ ⋆ ∈ [σ0, 1), which corresponds to the already found ρ ⋆ and satisfies condition (1.13). Let β = 3π 4 ρ ⋆ . Then formula (3.1) will have the form
It is not hard to verify, that g ′ 2 (λ) has the estimate (is proved in a completely similar way to estimate of J3)
Therefore, for all t0 > 1 we have
Hence this derivative is negative if
Thus, if λ1 ≥ Λ1, where (see Remark 1.7) (4.2) Λ1 = e n , n ≥ ln(5 Γ(1 − ρ ⋆ )) σ0 , then e λ σ 1 (ρ ⋆ ), as a function of σ ∈ [σ0, 1), strictly decreases for all t0 > 1. Moreover, for all λ1 ≥ Λ1 and σ ∈ [σ0, 1) the following estimate is fulfilled e λ 1 (ρ ⋆ ) < e λ σ 1 (ρ ⋆ ) < e1(ρ ⋆ ). The last estimate shows that if d1 satisfies condition (1.14), then, assuming ρ ⋆ has already been found, we can easily determine the parameter σ ⋆ from equality (1.13), that is, from e λ σ 1 (ρ ⋆ )|φ(ξ1)| = d1.
The authors convey thanks to Sh. A. Alimov for discussions of these results.