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Calculate accurately this hypergeometric function?

Posted 6 years ago

Hi, I have a problem with forcing MATHEMATICA to calculate accurately some hypergeometric function: s = 10^50; Block[{$MaxExtraPrecision = 10000},N[Hypergeometric2F1[2, 1 + s, 2 + s, -Exp[10]], 300]]. I am obtaining messages about overflow. The messages disappear when I request 15 digits, but I need 300 digits. It is hard to understand why the overflow arises; judging from the plot of the function, there seems to be a constant limit when s->Infinity. Increasing MaxExtraPrecision does not help. Strangely, Limit[Hypergeometric2F1[2, 1 + s, 2 + s, -Exp[10]],s->Infinity] does not work and it is also impossible to obtain an asymptotic series for s->Infinity by using the Series command. Is there any way to overcome the problem? Leslaw

POSTED BY: Leslaw Bieniasz
2 Replies

We have:

Sum[(z^k Pochhammer[2, k] Pochhammer[1 + s, k])/(
 k! Pochhammer[2 + s, k]), {k, 0, Infinity}]
 (*Hypergeometric2F1[2, 1 + s, 2 + s, z]*)

then:

 Limit[(z^k Pochhammer[2, k] Pochhammer[1 + s, k])/(k! Pochhammer[2 + s, k]), s -> Infinity]
 (*(z^k Pochhammer[2, k])/k!*) 
 N[Sum[(z^k Pochhammer[2, k])/k!, {k, 0, Infinity}] /. z -> -Exp[10], 300]

 (*2.06096648272347233418959215235994959191908111757487814505583864436199\
 9184683402296197924473085819095047447295634942724479179145245736116114\
 9156752936212085037037409690812216803036585395729723051186738618366655\
 5146641627747571896005193529868156699033219139667312892465093410814678\
 504338970396392816996*10^-9*)

$$\frac{1}{\left(1+e^{10}\right)^2}$$ Symbolic value what You are looking for.

POSTED BY: Mariusz Iwaniuk
POSTED BY: Mariusz Iwaniuk
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