OFFSET
1,5
FORMULA
a(n) = |{j < n : sigma(j) - j = sigma(n) - n}|.
MATHEMATICA
Table[Length[Select[Range[n - 1], DivisorSigma[1, #] - # == DivisorSigma[1, n] - n &]], {n, 91}]
PROG
(PARI) a(n)={my(t=sigma(n)-n); sum(k=1, n-1, sigma(k)-k==t)} \\ Andrew Howroyd, Oct 31 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Gutkovskiy, Oct 31 2020
STATUS
approved