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

%I A144551 #16 Jul 24 2024 17:33:05
%S A144551 0,2,12,24,36,45,60,84,105,120,144,180,210,231,264,300,325,351,378,
%T A144551 420,480,528,561,595,630,684,741,780,840,924,990,1035,1104,1176,1225,
%U A144551 1275,1326,1404,1485,1540,1596,1653,1740,1860,1953,2016,2080,2145,2244,2346
%N A144551 a(n) = nonprime(n)*nonprime(n+1)/2, where nonprime(n) = A141468(n).
%e A144551 a(1) = 0*1/2 = 0, a(2) = 1*4/2 = 2, a(3) = 4*6/2 = 12, etc.
%p A144551 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
%t A144551 (Times@@#)/2&/@Partition[Select[Range[0,100],!PrimeQ[#]&],2,1] (* _Harvey P. Dale_, Feb 12 2020 *)
%o A144551 (PARI) c(n) = {for(k=0, primepi(n), isprime(n++)&&k--); n};
%o A144551 t(n) = if(n<3,n-1,c(n-2));
%o A144551 vector(100, n, t(n)*t(n+1)/2) \\ _Altug Alkan_, Oct 17 2015
%o A144551 (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
%Y A144551 Cf. A141468.
%K A144551 nonn
%O A144551 1,2
%A A144551 _Juri-Stepan Gerasimov_, Dec 31 2008
%E A144551 1963 replaced by 1953 by _R. J. Mathar_, Jan 03 2009