A144551 a(n) = nonprime(n)*nonprime(n+1)/2, where nonprime(n) = A141468(n).

0, 2, 12, 24, 36, 45, 60, 84, 105, 120, 144, 180, 210, 231, 264, 300, 325, 351, 378, 420, 480, 528, 561, 595, 630, 684, 741, 780, 840, 924, 990, 1035, 1104, 1176, 1225, 1275, 1326, 1404, 1485, 1540, 1596, 1653, 1740, 1860, 1953, 2016, 2080, 2145, 2244, 2346

Offset: 1

Author

Juri-Stepan Gerasimov, Dec 31 2008

Keywords

• nonn

Examples

			a(1) = 0*1/2 = 0, a(2) = 1*4/2 = 2, a(3) = 4*6/2 = 12, etc.
		

Crossrefs

Cf. A141468.

Programs

Maple

A141468 := proc(n) option remember ; local a; if n = 1 then 0; else for a from procname(n-1)+1 do if not isprime(a) then RETURN(a) ; fi; od: fi; end: A144551 := proc(n) A141468(n)*A141468(n+1)/2 ; end: for n from 1 to 200 do printf("%d,",A144551(n)) ; od: # R. J. Mathar, Jan 03 2009

Mathematica

(Times@@#)/2&/@Partition[Select[Range[0,100],!PrimeQ[#]&],2,1] (* Harvey P. Dale, Feb 12 2020 *)

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

Extensions

1963 replaced by 1953 by R. J. Mathar, Jan 03 2009