OFFSET
1,2
COMMENTS
Sigma(n,k) is the sum of the k-th powers of the divisors of n.
The sequence seems to be strictly increasing. - Chayim Lowen, Aug 05 2015.
This is true. Moreover, subsequent ratios a(n+1)/a(n) steadily grow for n>3. The difference of subsequent ratios tends to the limit e = 2.718... The reason is that a(n) roughly behaves like n^n; already the second largest term in the sum is smaller by a factor 2^n (for even n) or by a factor 3^n (for n=6k+3) etc. - M. F. Hasler, Aug 16 2015
LINKS
Robert Israel, Table of n, a(n) for n = 1..353
FORMULA
a(n) = n + Sum_{d|n, d>1} d*(d^n-1)/(d-1). - Chayim Lowen, Aug 02 2015
a(n) >= n*(n^n+n-2)/(n-1) for n>1. - Chayim Lowen, Aug 05 2015
EXAMPLE
a(4) = sigma(4,1) + sigma(4,2) + sigma(4,3) + sigma(4,4) = 7 + 21 + 73 + 273 = 374.
MAPLE
seq(add(numtheory:-sigma[k](n), k=1..n), n=1..50); # Robert Israel, Aug 04 2015
MATHEMATICA
Table[Sum[DivisorSigma[i, n], {i, n}], {n, 19}] (* Michael De Vlieger, Aug 06 2015 *)
f[n_] := Sum[DivisorSigma[i, n], {i, n}]; (* OR *) f[n_] := Block[{d = Rest@Divisors@n}, n + Total[(d^(n + 1) - d)/(d - 1)]]; (* then *) Array[f, 19] (* Robert G. Wilson v, Aug 06 2015 *)
PROG
(PARI) vector(30, n, sum(k=1, n, sigma(n, k)))
(PARI) vector(30, n, n + sumdiv(n, d, if (d>1, (d^(n+1)-d)/(d-1)))) \\ Michel Marcus, Aug 04 2015
CROSSREFS
KEYWORD
nonn,easy,changed
AUTHOR
Colin Barker, Jan 22 2014
STATUS
approved