OFFSET
1,1
COMMENTS
Semiprime analog of A061712. Extended by Stefan Steinerberger. Includes the subset Mersenne semiprimes A092561.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..250
EXAMPLE
a(1) = 4 because the first semiprime A001358(1) is 4 (base 10) which is written 100 in binary, the latter representation having exactly 1 one.
a(2) = 6 since A001358(2) = 6 = 110 (base 2) has exactly 2 ones.
a(4) = 15 since A001358(6) = 15 = 1111 (base 2) has exactly 4 ones and, as it also has no zeros, is the smallest of the Mersenne semiprimes.
MATHEMATICA
Join[{4}, Table[SelectFirst[Sort[FromDigits[#, 2]&/@Permutations[ Join[ PadRight[{}, n, 1], {0}]]], PrimeOmega[#]==2&], {n, 2, 40}]] (* Harvey P. Dale, Feb 06 2015 *)
CROSSREFS
KEYWORD
easy,base,nonn
AUTHOR
Jonathan Vos Post, Jun 23 2007
STATUS
approved