A102415 - OEIS

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

(list; graph; refs; listen; history; text; internal format)

OFFSET

3,1

LINKS

Amiram Eldar, Table of n, a(n) for n = 3..10000

Eric Weisstein's World of Mathematics, Semiprime

FORMULA

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

EXAMPLE

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

MATHEMATICA

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