A102415 Greatest semiprime less than n-th prime.

4, 6, 10, 10, 15, 15, 22, 26, 26, 35, 39, 39, 46, 51, 58, 58, 65, 69, 69, 77, 82, 87, 95, 95, 95, 106, 106, 111, 123, 129, 134, 134, 146, 146, 155, 161, 166, 169, 178, 178, 187, 187, 194, 194, 209, 221, 226, 226, 226, 237, 237, 249, 254, 262, 267, 267, 274, 278, 278

Offset: 3

Author

Reinhard Zumkeller, Jan 08 2005

Keywords

• nonn

Examples

			a(3) = 4 since 4 is the greatest semiprime less than prime(3) = 5.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 3..10000
• Eric Weisstein's World of Mathematics, Semiprime

Crossrefs

Cf. A001358, A102414, A102440.

Programs

Mathematica

a[n_] := Module[{m = Prime[n] - 1}, While[PrimeOmega[m] != 2, m--]; m]; Array[a, 60, 3] (* Amiram Eldar, Feb 06 2020 *)

PARI

a(n) = {sp = prime(n)-1; while(bigomega(sp) != 2, sp--); sp;} \\ Michel Marcus, Mar 04 2017

Formula

a(n) < A000040(n) < A102414(n).