OFFSET
1,2
COMMENTS
This is the semiprime analog of A057704.
a(21) > 10000. - Tyler Busby, Feb 12 2023
EXAMPLE
4*6*9*10*14*15 - 1 = 453599 which is prime.
MATHEMATICA
NextSemiPrime[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sp = n + sgn; While[c < Abs[k], While[ PrimeOmega[sp] != 2, If[sgn < 0, sp--, sp++]]; If[sgn < 0, sp--, sp++]; c++]; sp + If[sgn < 0, 1, -1]]; f[n_] := Times @@ NestList[NextSemiPrime, 2^2, n - 1]; k = 1; lst = {}; While[k < 3501, If[ PrimeQ[f[k] - 1], Print[k]; AppendTo[lst, k]]; k++]; lst
PROG
(PARI) s=3; t=1; for(n=1, 1000, while(bigomega(s++)!=2, ); t*=s; if(ispseudoprime(t-1), print1(n", "))) \\ Charles R Greathouse IV, Apr 03 2013
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Jonathan Vos Post and Robert G. Wilson v, Apr 02 2013
EXTENSIONS
a(19) from Charles R Greathouse IV, Apr 03 2013
a(20) from Tyler Busby, Feb 12 2023
STATUS
approved