OFFSET
1,1
LINKS
David A. Corneth, Table of n, a(n) for n = 1..10000
EXAMPLE
a(1)=4 because 5*2=(2*2)-th semiprime, a(2)=6 because 5*3=(2*3)-th semiprime, a(3)=10 because 13*2=(5*2)-th semiprime, a(4)=14 because 19*2=(7*2)-th semiprime, a(5)=15 because 13*3=(5*3)-th semiprime, a(6)=22 because 31*2=(11*2)-th semiprime.
MAPLE
Contribution from R. J. Mathar, Aug 10 2010: (Start)
A001358 := proc(n) option remember; local a; if n = 1 then 4; else for a from procname(n-1)+1 do if numtheory[bigomega](a) = 2 then return a; end if; end do: end if; end proc:
MATHEMATICA
SemiPrimeQ[n_Integer] := If[Abs[n]<2, False, (2==Plus@@Transpose[FactorInteger[Abs[n]]][[2]])]; sp=Select[Range[10000], SemiPrimeQ]; sp2=Select[sp, #<Length[sp]&]; Select[sp2, GCD[ #, sp[[ # ]]]>1&] (* T. D. Noe, Aug 11 2010 *)
PROG
(PARI) first(n) = {my(t = 0, res = List()); for(i = 1, oo, if(bigomega(i) == 2, t++; if(bigomega(t) == 2 && gcd(t, i) > 1, listput(res, t); if(#res >= n, return(res) ) ) ); ); res } \\ David A. Corneth, Mar 15 2023
CROSSREFS
KEYWORD
nonn,less
AUTHOR
Juri-Stepan Gerasimov, Aug 10 2010
EXTENSIONS
Corrected and extended by R. J. Mathar and T. D. Noe, Aug 11 2010
STATUS
approved