OFFSET
1,2
EXAMPLE
a(1) = 0*1/2 = 0, a(2) = 1*4/2 = 2, a(3) = 4*6/2 = 12, etc.
MAPLE
MATHEMATICA
(Times@@#)/2&/@Partition[Select[Range[0, 100], !PrimeQ[#]&], 2, 1] (* Harvey P. Dale, Feb 12 2020 *)
PROG
(PARI) c(n) = {for(k=0, primepi(n), isprime(n++)&&k--); n};
t(n) = if(n<3, n-1, c(n-2));
vector(100, n, t(n)*t(n+1)/2) \\ Altug Alkan, Oct 17 2015
(PARI) a(n) = my(A141468(n)=my(k=0); n--; while(-n+n+=-k+k=primepi(n), ); n); A141468(n)*A141468(n+1)/2; \\ Ruud H.G. van Tol, Jul 15 2024
CROSSREFS
KEYWORD
nonn
AUTHOR
Juri-Stepan Gerasimov, Dec 31 2008
EXTENSIONS
1963 replaced by 1953 by R. J. Mathar, Jan 03 2009
STATUS
approved