OFFSET
1,1
COMMENTS
Primes of the form 2^n+1, i.e., Fermat primes (A019434) are terms of this sequence.
For n > 32, a(n) > 2 * 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, 2k - DivisorSigma[1, k]] != n, k++]; k, {n, 31}]
PROG
(PARI) a(n) = my(k=1); while(2*k - sigma(k) != 2^n, k++); k; \\ Michel Marcus, Sep 19 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
XU Pingya, Sep 19 2017
STATUS
approved