1F1(a;r;z)1F1(a;r;-z)=2F3(a,r-a;r,r/2,r/2+1/2;z^2/4)

Theorem

The following formula holds: $${}_1F_1(a;r;z){}_1F_1(a;r;-z)={}_2F_3\left(a, r-a; r, \dfrac{r}{2}, \dfrac{r}{2}+\dfrac{1}{2}; \dfrac{z^2}{4} \right),$$ where ${}_1F_1$ denotes hypergeometric 1F1 and ${}_2F_3$ denotes hypergeometric 2F3.

References

• 1953: Arthur Erdélyi, Wilhelm Magnus, Fritz Oberhettinger and Francesco G. Tricomi: Higher Transcendental Functions Volume I ... (previous) ... (next): $4.2 (5)$