OFFSET
1,3
COMMENTS
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..10000
FORMULA
a(A007369(n)) = 0.
EXAMPLE
There are no integers such that sigma(x)=2, so a(2)=0.
There is a single integer, x=2, such that sigma(x)=3, so a(3)=2.
There are 2 integers, x=6 and 11, such that sigma(x)=12, their gcd is 1, so a(12)=1.
MAPLE
A240667 := n -> igcd(op(select(k->sigma(k)=n, [$1..n]))):
seq(A240667(n), n=1..82); # Peter Luschny, Apr 13 2014
MATHEMATICA
a[n_] := GCD @@ Select[Range[n], DivisorSigma[1, #] == n&];
Array[a, 100] (* Jean-François Alcover, Jul 30 2018 *)
PROG
(PARI) sigv(n) = select(i->sigma(i) == n, vector(n, i, i));
a(n) = {v = sigv(n); if (#v == 0, 0, gcd(v)); }
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Marcus, Apr 10 2014
STATUS
approved