login
A292558
a(n) is the smallest number k such that sigma(k) - 2k = 2^n.
1
20, 12, 56, 550, 572, 108, 860, 952, 1232, 6328, 3708, 40540, 37072, 79288, 327260, 357112, 302000, 527296, 1764056, 6506512, 38559776, 21893248, 42257216, 167771740, 90798560, 469761208, 508198064, 490304800, 1353048560, 2951488480, 5067417200
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
KEYWORD
nonn
AUTHOR
XU Pingya, Sep 19 2017
STATUS
approved