OFFSET
3,1
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 3..10000
FORMULA
a(n) = MAX{k in A001358 and k < n^2}.
EXAMPLE
Offset is 3 because there is no semiprime less than 2^2 = 4 (as 4 is the smallest semiprime).
a(3) = 6 because 6 is the largest semiprime less than 3^2 = 9 (itself a semiprime), with only the prime 7 and the triprime 8 properly in the [6,9] interval.
a(4) = 15 < 16 = 4^2.
MATHEMATICA
semiPrimeQ[n_] := Total[FactorInteger[n]][[2]] == 2; Table[k = n^2 - 1; While[! semiPrimeQ[k], k--]; k, {n, 3, 100}] (* T. D. Noe, Mar 15 2011 *)
PROG
(PARI) issemi(n)=bigomega(2)==2
a(n)=n*=n; while(!issemi(n--), ); n \\ Charles R Greathouse IV, Mar 16 2011
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jonathan Vos Post, Mar 15 2011
STATUS
approved