OFFSET
1,2
COMMENTS
Conjecture: for n > 1, a(n) is of the form 2^n * m generally, sometimes of the form 3^n * m, and sometimes of the form 2^(n-1) * m, depending on sigma(m). Upper bounds are mostly of the form 2^n * m for odd m. For example, a(27) <= 2^27 * 5. - David A. Corneth, Feb 14 2019
LINKS
David A. Corneth, PARI program for some upper bounds below specified value
David A. Corneth, Upper bounds (or actual values) for a(n)
PROG
(PARI) fk(s, m) = {my(j = 1); while(denominator(s^j/m) != 1, j++); j; }
rad(n) = factorback(factorint(n)[, 1]);
a(n) = {my(k = 1, ok = 0, sk); while (!ok, sk = sigma(k); if ((denominator(sk/rad(k)) == 1) && (fk(sk, k) == n), ok = 1, k++; ); ); k; } \\ corrected by Michel Marcus, Feb 14 2019
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Michel Marcus, Nov 06 2015
STATUS
approved