OFFSET
1,2
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
FORMULA
Logarithmic derivative of A103446 (with offset=0), which describes the binomial transform of partitions.
From Paul D. Hanna, Jun 01 2013: (Start)
L.g.f.: Sum_{n>=1} sigma(n) * x^n/(1-x)^n / n.
L.g.f.: Sum_{n>=1} x^n/((1-x)^n - x^n) / n.
L.g.f.: Sum_{n>=1} n*log(1-x) - log((1-x)^n - x^n).
L.g.f.: Sum_{n>=1} A001511(n) * log(1 + x^n/(1-x)^n), where 2^A001511(n) is the highest power of 2 that divides 2*n.
a(n) = A222115(n) - 1. (End)
a(n) ~ Pi^2/12 * n * 2^n. - Vaclav Kotesovec, Dec 30 2015
a(n) = Sum_{i=1..n} Sum_{j=1..n} i*binomial(n,i*j). - Ridouane Oudra, Nov 12 2019
EXAMPLE
L.g.f.: L(x) = x + 5*x^2/2 + 16*x^3/3 + 45*x^4/4 + 116*x^5/5 + 284*x^6/6 +...
where exponentiation yields A103446 (with offset=0):
exp(L(x)) = 1 + x + 3*x^2 + 8*x^3 + 21*x^4 + 54*x^5 + 137*x^6 + 344*x^7 +...
MAPLE
with(numtheory): seq(add(binomial(n, i)*sigma(i), i=1..n), n=1..40); # Ridouane Oudra, Nov 12 2019
MATHEMATICA
Table[Sum[Binomial[n, k] DivisorSigma[1, k], {k, n}], {n, 50}] (* G. C. Greubel, Jun 03 2017 *)
PROG
(PARI) {a(n)=sum(k=1, n, sigma(k)*binomial(n, k))}
for(n=1, 30, print1(a(n), ", "))
(PARI) {a(n)=local(X=x+x*O(x^n)); n*polcoeff(sum(m=1, n+1, x^m/((1-x)^m-X^m)/m), n)}
(PARI) {a(n)=local(X=x+x*O(x^n)); n*polcoeff(sum(k=1, n, k*log(1-X)-log((1-x)^k-X^k)), n)}
(PARI) {a(n)=local(X=x+x*O(x^n)); n*polcoeff(sum(m=1, n+1, sigma(m)*x^m/(1-X)^m/m), n)}
(PARI) {a(n)=local(X=x+x*O(x^n)); n*polcoeff(sum(k=1, n, valuation(2*k, 2)*log(1 + x^k/(1-X)^k)), n)}
(Magma) [&+[Binomial(n, k)*DivisorSigma(1, k):k in [1..n]]:n in [1..31]]; // Marius A. Burtea, Nov 12 2019
(Magma) [&+[&+[i*Binomial(n, i*j):j in [1..n]]:i in [1..n]]:n in [1..31]]; // Marius A. Burtea, Nov 12 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Paul D. Hanna, Feb 04 2012
STATUS
approved