OFFSET
0,3
COMMENTS
LINKS
Amiram Eldar, Table of n, a(n) for n = 0..300
Max Alekseyev, PARI/GP Scripts for Miscellaneous Math Problems (invphi.gp).
EXAMPLE
a(0) = 1 because 1 is the smallest number k with sigma(1) = 1 = 2^0.
a(5) = 21 because 21 is the smallest number k with sigma(k) = 32 = 2^5.
a(6) = 0 because there is no number k with sigma(k) = 64 = 2^6.
PROG
(PARI) a(n) = for (k=1, 2^n, if (sigma(k)== 2^n, return (k))); return (0); \\ Michel Marcus, Oct 03 2014, Oct 31 2015
(PARI) a(n) = max(0, invsigmaMin(1<<n)); \\ Amiram Eldar, Dec 31 2024, using Max Alekseyev's invphi.gp (see links).
CROSSREFS
KEYWORD
nonn
AUTHOR
Jaroslav Krizek, Sep 28 2014
EXTENSIONS
a(0) = 1 prepended by Michel Marcus, Oct 31 2015
STATUS
approved