OFFSET
1,2
COMMENTS
EXAMPLE
For every prime p the sum of proper divisors of p is 1, and the GCD of the primes is 1, so a(1) = 1.
PROG
(PARI) build(nb) = {vs = vector(nb); nc = nb^2; forcomposite(n=2, nc, val = sigma(n)-n; if (val <= nb, if (vs[val] == 0, vs[val] = -n, vs[val] = gcd(vs[val], n)); ); ); vs[1] = 1; vs; }
lista() = {vd = build(100000); vmax = 20; vr = vector(vmax); for (ig=1, vmax, for (i=1, #vd, w = vd[i]; if (w ==ig, vr[ig] = i; break; ); ); ); for (i=1, #vr, if (vr[i] == 0, break, print1(vr[i], ", "))); }
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Michel Marcus, Dec 30 2014
EXTENSIONS
a(1)=1 suggested by Robert Israel, Jan 01 2015
STATUS
approved