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 A115392 First appearance of n-th prime as prime factor in list of semiprimes. 26
 1, 2, 4, 5, 8, 10, 12, 14, 16, 21, 22, 25, 27, 29, 33, 35, 38, 41, 45, 47, 50, 52, 55, 58, 62, 64, 67, 70, 73, 76, 82, 84, 87, 88, 93, 96, 100, 104, 107, 111, 113, 115, 120, 121, 124, 126, 133, 137, 141, 142, 143, 147, 149, 155, 158, 162, 168, 169, 174, 176, 178, 183 (list; graph; refs; listen; history; text; internal format)
 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 A001358(a(n)) = 2*A000040(n) - Zak Seidov, Jun 27 2017 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 Cf. A000040, A001358. Sequence in context: A005242 A323976 A188975 * A342541 A179509 A157007 Adjacent sequences:  A115389 A115390 A115391 * A115393 A115394 A115395 KEYWORD nonn AUTHOR Zak Seidov, Mar 08 2006 EXTENSIONS Edited by Zak Seidov, Jun 27 2017 STATUS approved

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Last modified August 14 17:36 EDT 2022. Contains 356122 sequences. (Running on oeis4.)