OFFSET
1,1
COMMENTS
This is to A007647 as semiprimes A001358 is to primes A000040. The corresponding semiprimes are A216376 = {201, 500001, 130000000000001, 280000000000000000000000000001, ...}.
a(14) >= 414. - Daniel Suteu, Jul 09 2019
EXAMPLE
a(1) = 2 because 2 * 10^2 + 1 = 201 = 3 * 67.
a(2) = 5 because 5 * 10^5 + 1 = 500001 = 3 * 166667.
a(3) = 13 because 13*10^13 + 1 = 130000000000001 = 6529 * 19911165569.
a(4) = 28 because 28 * 10^28 + 1 = 29 * 9655172413793103448275862069.
MATHEMATICA
Select[Range[40], PrimeOmega[# 10^# + 1] == 2 &] (* Alonso del Arte, Sep 08 2012 *)
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..70] | IsSemiprime(s) where s is n*10^n+1]; // Vincenzo Librandi, May 10 2014
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Jonathan Vos Post, Sep 06 2012
EXTENSIONS
a(8)-a(13) from Daniel Suteu, Jul 09 2019
STATUS
approved