cp's OEIS Frontend

A141556 Composites of the form c(p(n)) + p(c(n)), where c(n) = n-th composite and p(n) = n-th prime.

%I A141556 #13 Jan 29 2023 08:01:42
%S A141556 21,49,70,77,88,105,117,176,185,192,205,236,247,292,301,309,323,345,
%T A141556 365,394,405,411,427,435,455,478,490,501,513,538,554,567,585,622,636,
%U A141556 640,655,675,713,747,759,767,785,794,833,845,854,862,891,905,921,933,978
%N A141556 Composites of the form c(p(n)) + p(c(n)), where c(n) = n-th composite and p(n) = n-th prime.
%e A141556 For n=1, c(1)= 4, p(1)= 2; c(2)  + p(4)  =  6 +  7 = 13 (prime).
%e A141556 For n=2, c(2)= 6, p(2)= 3; c(3)  + p(6)  =  8 + 13 = 21 = a(1).
%e A141556 For n=3, c(3)= 8, p(3)= 5; c(5)  + p(8)  = 10 + 19 = 29 (prime).
%e A141556 For n=4, c(4)= 9, p(4)= 7; c(7)  + p(9)  = 14 + 23 = 37 (prime).
%e A141556 For n=5, c(5)=10, p(5)=11; c(11) + p(10) = 20 + 29 = 49 = a(2).
%e A141556 For n=6, c(6)=12, p(6)=13; c(13) + p(12) = 22 + 37 = 59 (prime).
%e A141556 For n=7, c(7)=14, p(7)=17; c(17) + p(14) = 27 + 43 = 70 = a(3).
%o A141556 (PARI) p(n) = prime(n); \\ A000040
%o A141556 c(n) = for(k=0, primepi(n), isprime(n++)&&k--); n; \\ A002808
%o A141556 select(x->(!isprime(x)), vector(70, n, c(p(n)) + p(c(n)))) \\ _Michel Marcus_, Jan 29 2023
%Y A141556 Cf. A000040, A002808, A141555.
%K A141556 nonn
%O A141556 1,1
%A A141556 _Juri-Stepan Gerasimov_, Aug 14 2008
%E A141556 Edited and corrected by _Ray Chandler_, Aug 19 2008