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Calculate the following hypergeometric series?

Posted 5 years ago

Hi, I am trying to perform the command:

Series[HypergeometricPFQ[{1,1,1,1},{2,2,2,2},-x],{x,Infinity,5}] 

using Wolfram Alpha, but I am confused by the results returned. There are three different answers displayed. The first says that no series expansion is available. The second answer is a formula involving some logarithmic terms plus exp(-x) multiplied by some power series of variable 1/x. The third answer is a compact formula for a power series of variable 1/x, apparently different from the previous series. So, which of these answers is correct? Leslaw

POSTED BY: Leslaw Bieniasz
6 Replies

I think you miss the point. Surely, for a suitably chosen value of x, you can obtain a reasonably accurate approximation to the value of HypergeometricPFQ, either by using the MATHEMATICA built-in procedure, or by the Taylor series around x=0, or by the asymptotic expansion around x=Infinity. (In fact, the Taylor series is convergent for any x in this case, and I suspect MATHEMATICA uses it for calculating HypergeometricPFQ, although this is certainly inefficient for large x, which manifests itself by numerical difficulties and increasing computational times for large x). However, I am trying to draw your attention to a different fact, namely that response 3 of Wolfram Alpha is NOT the asymptotic expansion for which I asked (by calling Series with {x,Infinity,5}, although it is returned as such. This is a mistake which should be corrected. Whether or not the three methods/formulae yield nearly the same numerical result does not matter. Leslaw.

POSTED BY: Leslaw Bieniasz

I doubt it was a mistake. W|A often returns more than was requested. Some users like that, actually.

Enough time spent on this. Over and out.

POSTED BY: Daniel Lichtblau

One more comment: When someone enters a command to obtain a series expansion around Infinity, then it should be obvious that one wants to obtain an asymptotic expansion, not a Taylor series. In such a situation, the returned answer 1: "no series expansion available" is simply misleading and should also not be displayed by Wolfram Alpha. It is also inconsistent with answer 2. Leslaw

POSTED BY: Leslaw Bieniasz

I do not see any possibility for the equivalence of series 2 and 3. Series 3 assumes that n<=0 which implies that x^(-n) would mean only nonnegative powers of x. In contrast, in series 2 we have only negative powers of x (as is expected for an asymptotic series) plus some logarithms, which are unlikely to have asymptotic expansions in the form of negative powers of x. Hence, I think answer 3 is nonsensical. In fact, series 3 seems to be obtained by a simple replacement n->-n in a Taylor series expansion around x=0. However, in such a case series 3 should not be presented by the Wolfram Alpha interface as an expansion around infinity. Taking into account the fact that thousands of uneducated students around the world frantically use Wolfram Alpha and uncritically believe in the results it returns, this mistake should be corrected as soon as possible, Leslaw

POSTED BY: Leslaw Bieniasz

Did you test this? At all? (These are rhetorical questions.)

Here are the relevant computations.

ee = HypergeometricPFQ[{1, 1, 1, 1}, {2, 2, 2, 2}, -x];

First the series from the second pod.

ss = Normal[Series[ee, {x, Infinity, 5}]]

(* Out[477]= 
E^-x (-(6/x^5) + 1/x^4) + (1/(
 12 x))(2 EulerGamma^3 + EulerGamma \[Pi]^2 + 
   6 EulerGamma^2 Log[x] + \[Pi]^2 Log[x] + 6 EulerGamma Log[x]^2 + 
   2 Log[x]^3 - 2 PolyGamma[2, 1]) *)

Quick numerical check.

{ee, ss} /. x -> 1111.1

(* Out[479]= {0.0715749986818, 0.0715749986818} *)

Good so far. Now for that third pod. I rewrote ever so slightly to use the more common positive exponent notation.

ss2 =  Sum[(-1)^n*x^n/((n + 1)^4*Factorial[n]), {n, 0, Infinity}]
ss2 === ee

(* Out[480]= HypergeometricPFQ[{1, 1, 1, 1}, {2, 2, 2, 2}, -x]

Out[481]= True *)

To restate, it automatically evaluated to the input expression. We can check numerically that this is plausible.

terms = 4000;
NSum[(-1)^n*(1111 + 1/10)^n/((n + 1)^4*Factorial[n]), {n, 0, terms}, 
 WorkingPrecision -> 1000, NSumTerms -> terms]

(* Out[504]= 
0.07157499868179492615228113024556896152461872165957332195911424357595\
7027394013518883378928089038297513792617279360234433705402207531750966\
0717006728987049036451842823432573305827850163550084804675769957660934\
4824128631596694605973285589988236274789694902987106035520518673838341\
8403680202985945005798435970592342850033187925346634452756821088978508\
5259444658718487981866392080437473574473862047376793571126464969800787\
8803181449171740731887666924329532184773807981764974033314775480749947\
1096391441816719897059492637173806116477129407659923386426954912528103\
9169790296933292120323513441841312866076886262356996247624553397188136\
7527417883237598541289814556372772783972646774583249135592744712362526\
9777934478025389513686897128733758151270977948985716655740364364858294\
3580719151581187700654582635529859694967679171786536858445379972974013\
6089392197806995711217621078952538615859473022861414912694258074963677\
5180352839003018355521752110323676919920539509259744224721471269246962\
90624132618936092170922 *)

(Exercise: Show that 4000 terms suffices to get a good approximation.)

Or use Sum with a numeric input for x:

Sum[(-1)^n*(1111.1)^n/((n + 1)^4*Factorial[n]), {n, 0, Infinity}]

(* Out[495]= 0.0715749986818 *)

Since careful verification is the order of the day, I'll show the variant from the W|A third pod as well:

Sum[(-1)^(-n)*x^(-n)/((n - 1)^4*Factorial[-n]), {n,  0, -Infinity, -1}]

(* Out[505]= HypergeometricPFQ[{1, 1, 1, 1}, {2, 2, 2, 2}, -x] *)

Regarding the nature of series expansions: many functions have Puiseux expansions at infinity. This one does not. I agree the first pod either should not be present, or should state more clearly what is intended. But that does not make it wrong, just a bit confusing when taken in tandem with the next two.

POSTED BY: Daniel Lichtblau

They might all be correct, suitably interpreted. There is no actual power series. The second and third are not power series per se. Quite possibly they are equivalent. Best I can suggest would be to try numeric tests.

POSTED BY: Daniel Lichtblau
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