A324600 a(n) = (k(n)*(k(n) + 1))/2 with k = A018252 (nonprime numbers), for n >= 1.

1, 10, 21, 36, 45, 55, 78, 105, 120, 136, 171, 210, 231, 253, 300, 325, 351, 378, 406, 465, 528, 561, 595, 630, 666, 741, 780, 820, 903, 990, 1035, 1081, 1176, 1225, 1275, 1326, 1378, 1485, 1540, 1596, 1653, 1711, 1830, 1953, 2016, 2080, 2145, 2211, 2346, 2415, 2485, 2628, 2775, 2850, 2926, 3003, 3081, 3240, 3321, 3403, 3570, 3655, 3741, 3828, 3916

Offset

1,2

Comments

This is a proper subsequence of the triangular numbers A000217.

The infinite sum of the reciprocals is given in A307379 (which motivated this entry).

Links

Table of n, a(n) for n=1..65.

Formula

a(n) = A000217(A018252(n)), for n >= 1.

Prog

(Magma) v:=[m:m in [1..500]| not IsPrime(m)]; [v[k]*(v[k]+1)/2:k in [1..65]]; // Marius A. Burtea, Jul 12 2019
(Magma) [n*(n+1)/2 : n in [1..100] | not IsPrime(n)]; // Vincenzo Librandi, Jul 12 2019
(PARI) apply(x->x*(x+1)/2, select(x->!isprime(x), [1..90])) \\ Michel Marcus, Sep 18 2023

Crossrefs

Cf. A000217, A018252, A034953 (for prime numbers).

Sequence in context: A082581 A075846 A164714 * A240536 A060852 A345962

Adjacent sequences: A324597 A324598 A324599 * A324601 A324602 A324603

Keyword

nonn,easy

Author

Wolfdieter Lang, Jul 10 2019

Status

approved