G. Gasper Using sums of squares to prove that ... - Fuchs-braun.com

and hence, by (3.7),
(4.14) | 1 F 1 (a; c; z)| 2 =
∞∑
k=0
(a) k (c − a) k
k! (c) 2k (c + k − 1) k
(−1) k y 2k
× 2 F 1 (−k, 1 − k; c; 1 + x 2 /y 2 )( 1 F 1 (a + k; c + 2k; x)) 2 .
Then differentiation of equation (4.13) with respect of y and application of
(3.7) gives the following extensions of (4.9) and (4.10) (and also of (3.19) and
(3.20)), respectively,
(4.15)
and
y ∂ ∂y |c(c + 1) 1F 1 (a; c; z)| 2 = 2y 2 ∞ ∑
(4.16)
∂ 2
k=0
(a) k+1 (c − a) k+1 (c + 1)
k! (c + 2) 2k (c + k + 1) k
(−1) k+1 y 2k
× 2 F 1 (−k, −k; c + 1; 1 + x 2 /y 2 )( 1 F 1 (a + k + 1; c + 2k + 2; x)) 2
∂y 2 |c(c + 1) 1F 1 (a; c; z)| 2 = 2
∞∑
k=0
(a) k+1 (c − a) k+1 (c + 1)
k! (c + 2) 2k (c + k + 1) k
(−1) k+1 y 2k
× 2 F 1 (−k, −k; c + 1; 1 + x 2 /y 2 )( 1 F 1 (a + k + 1; c + 2k + 2; x)) 2
+ 4y 2 ∞ ∑
k=0
(a) k+2 (c − a) k+2
k! (c + 2) 2k+2 (c + k + 3) k
(−1) k y 2k
× 2 F 1 (−k, −k − 1; c + 2; 1 + x 2 /y 2 )( 1 F 1 (a + k + 2; c + 2k + 4; x)) 2 .
If a = −n is a negative integer and c = α + 1, then (4.13)–(4.16) reduce
to (4.5), (4.6), (4.9), (4.10), respectively. If a = c + n with n a nonnegative
integer, then (4.15) and (4.16) reduce to terminating sums of squares expansions
with nonnegative coefficients which prove that c(c + 1) 1 F 1 (c + n; c; z),
as a function of z, has only real zeros when c ≥ −1, where this function is
to be replaced by its c → 0 and c → −1 limit cases when c = 0 and c = −1,
respectively. It should be noted that, in view of Kummer’s transformation
formula [9, 6.3(7)]
(4.17)
1F 1 (a; c; x) = e x 1F 1 (c − a; c; −x),
these results on the zeros of c(c + 1) 1 F 1 (c + n; c; z) are equivalent to those
obtained above for the Laguerre polynomials.
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