OFFSET
1,1
COMMENTS
For n > 31, a(n) > 1.724 * 10^10.
EXAMPLE
sigma(20) - 2*20 = 2^1, a(1) = 20.
sigma(108) - 2*108 = 64 = 2^6, a(6) = 108.
MATHEMATICA
Table[k = 1; While[Log[2, DivisorSigma[1, k] - 2k] != n, k++]; k, {n, 30}]
PROG
(PARI) a(n) = my(k=1); while(sigma(k) - 2*k != 2^n, k++); k; \\ Michel Marcus, Sep 19 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
XU Pingya, Sep 19 2017
STATUS
approved