login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A323570 G.f.: Sum_{n>=0} ((1+x)^n + i)^n / (2 + i*(1+x)^n)^(n+1), where i^2 = -1. 3
1, 5, 90, 2782, 120746, 6745934, 460888652, 37224433556, 3469644818024, 366563797094652, 43286560860984450, 5649978863795503006, 807731962396091271434, 125520224606849421561430, 21066610381145458077037434, 3797680233697346232224683014, 731834222599438183659030765632, 150129114941670825877326504381416, 32664492662327616894474980982618994, 7513192417898597095586779634504947174, 1821542500014252695637241514145496199576 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

It is remarkable that the generating function results in a power series in x with only real coefficients.

LINKS

Table of n, a(n) for n=0..20.

FORMULA

G.f.: Sum_{n>=0} ((1+x)^n + i)^n / (2 + i*(1+x)^n)^(n+1).

G.f.: Sum_{n>=0} ((1+x)^n - i)^n / (2 - i*(1+x)^n)^(n+1).

G.f.: Sum_{n>=0} ((1+x)^n + i)^n * (2 - i*(1+x)^n)^(n+1) / (4 + (1+x)^(2*n))^(n+1).

G.f.: Sum_{n>=0} ((1+x)^n - i)^n * (2 + i*(1+x)^n)^(n+1) / (4 + (1+x)^(2*n))^(n+1).

EXAMPLE

G.f.: A(x) = 1 + 5*x + 90*x^2 + 2782*x^3 + 120746*x^4 + 6745934*x^5 + 460888652*x^6 + 37224433556*x^7 + 3469644818024*x^8 + 366563797094652*x^9 + ...

such that

A(x) = 1/(2+i) + ((1+x) + i)/(2 + i*(1+x))^2 + ((1+x)^2 + i)^2/(2 + i*(1+x)^2)^3 + ((1+x)^3 + i)^3/(2 + i*(1+x)^3)^4 + ((1+x)^4 + i)^4/(2 + i*(1+x)^4)^5 + ((1+x)^5 + i)^5/(2 + i*(1+x)^5)^6 + ((1+x)^6 + i)^6/(2 + i*(1+x)^6)^7 + ...

also

A(x) = 1/(2-i) + ((1+x) - i)/(2 - i*(1+x))^2 + ((1+x)^2 - i)^2/(2 - i*(1+x)^2)^3 + ((1+x)^3 - i)^3/(2 - i*(1+x)^3)^4 + ((1+x)^4 - i)^4/(2 - i*(1+x)^4)^5 + ((1+x)^5 - i)^5/(2 - i*(1+x)^5)^6 + ((1+x)^6 - i)^6/(2 - i*(1+x)^6)^7 + ...

RELATED INFINITE SERIES.

At x = -1/2, the g.f. as a power series in x diverges, but the related series converges:

S = Sum_{n>=0} (1/2^n + i)^n / (2 + i/2^n)^(n+1).

Equivalently,

S = Sum_{n>=0} 2^n * (1 + 2^n*i)^n / (2^(n+1) + i)^(n+1) ;

written explicitly,

S = 1/(2+i) + 2*(1+2*i)/(2^2+i)^2 + 2^2*(1+2^2*i)^2/(2^3+i)^3

+ 2^3*(1+2^3*i)^3/(2^4+i)^4 + 2^4*(1+2^4*i)^4/(2^5+i)^5

+ 2^5*(1+2^5*i)^5/(2^6+i)^6 + 2^6*(1+2^6*i)^6/(2^7+i)^7 + ...

which equals the real number

S = 0.51940468012345114278789093541014909064793642824169972277862...

PROG

(PARI) {a(n) = my(A = sum(m=0, n*10 + 400, ((1+x +x*O(x^n))^m + I)^m/(2 + I*(1+x +x*O(x^n))^m)^(m+1)*1. )); round(polcoeff(A, n))}

for(n=0, 20, print1(a(n), ", "))

(PARI) {a(n) = my(A = sum(m=0, n*10 + 400, ((1+x +x*O(x^n))^m - I)^m/(2 - I*(1+x +x*O(x^n))^m)^(m+1)*1. )); round(polcoeff(A, n))}

for(n=0, 20, print1(a(n), ", "))

CROSSREFS

Cf. A323571, A323688.

Cf. A323681.

Sequence in context: A301359 A037297 A277303 * A295766 A216693 A158073

Adjacent sequences:  A323567 A323568 A323569 * A323571 A323572 A323573

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Feb 10 2019

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified September 20 16:50 EDT 2021. Contains 347586 sequences. (Running on oeis4.)