A141556 Composites of the form c(p(n)) + p(c(n)), where c(n) = n-th composite and p(n) = n-th prime.
21, 49, 70, 77, 88, 105, 117, 176, 185, 192, 205, 236, 247, 292, 301, 309, 323, 345, 365, 394, 405, 411, 427, 435, 455, 478, 490, 501, 513, 538, 554, 567, 585, 622, 636, 640, 655, 675, 713, 747, 759, 767, 785, 794, 833, 845, 854, 862, 891, 905, 921, 933, 978
Offset: 1
Keywords
Examples
For n=1, c(1)= 4, p(1)= 2; c(2) + p(4) = 6 + 7 = 13 (prime). For n=2, c(2)= 6, p(2)= 3; c(3) + p(6) = 8 + 13 = 21 = a(1). For n=3, c(3)= 8, p(3)= 5; c(5) + p(8) = 10 + 19 = 29 (prime). For n=4, c(4)= 9, p(4)= 7; c(7) + p(9) = 14 + 23 = 37 (prime). For n=5, c(5)=10, p(5)=11; c(11) + p(10) = 20 + 29 = 49 = a(2). For n=6, c(6)=12, p(6)=13; c(13) + p(12) = 22 + 37 = 59 (prime). For n=7, c(7)=14, p(7)=17; c(17) + p(14) = 27 + 43 = 70 = a(3).
Programs
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PARI
p(n) = prime(n); \\ A000040 c(n) = for(k=0, primepi(n), isprime(n++)&&k--); n; \\ A002808 select(x->(!isprime(x)), vector(70, n, c(p(n)) + p(c(n)))) \\ Michel Marcus, Jan 29 2023
Extensions
Edited and corrected by Ray Chandler, Aug 19 2008