OFFSET
1,2
COMMENTS
The positions of even semiprimes in A001358. - Juri-Stepan Gerasimov, Apr 11 2010
LINKS
Zak Seidov, Table of n, a(n) for n = 1..1000
FORMULA
EXAMPLE
a(5)=8 because 5th prime, 11, first appeared as a prime factor in 8th semiprime 22=2*11;
a(6)=10 because 6th prime, 13, first appeared as a prime factor in 10th semiprime 26=2*13.
MATHEMATICA
sp = Select[Range[4, 20000], 2 == PrimeOmega[#]&]; Table[Position[Mod[sp, Prime[k]], 0][[1, 1]], {k, 1000}] (* For first 1000 terms. - Zak Seidov, Jun 27 2017 *)
PROG
(PARI) first(n) = my(l = List([4, 1]), u = 2*prime(n), res = vector(n), t=0); forprime(p = 2, t++; sqrt(2*prime(n)), forprime(q = p+1, u\p, listput(l, [p*q, t])); listsort(l); for(i=1, #l, if(res[l[i][3]]==0, res[l[i][3]] = i)) \\ David A. Corneth, Jun 28 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Zak Seidov, Mar 08 2006
EXTENSIONS
Edited by Zak Seidov, Jun 27 2017
STATUS
approved