A242121 a(n) = n-th prime modulo 21.

2, 3, 5, 7, 11, 13, 17, 19, 2, 8, 10, 16, 20, 1, 5, 11, 17, 19, 4, 8, 10, 16, 20, 5, 13, 17, 19, 2, 4, 8, 1, 5, 11, 13, 2, 4, 10, 16, 20, 5, 11, 13, 2, 4, 8, 10, 1, 13, 17, 19, 2, 8, 10, 20, 5, 11, 17, 19, 4, 8, 10, 20, 13, 17, 19, 2, 16, 1, 11, 13, 17, 2, 10

Offset: 1

Author

Vincenzo Librandi, May 05 2014

Keywords

• nonn
• easy

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. similar sequences listed in A242119.

Programs

Magma

[p mod(21): p in PrimesUpTo(500)];

Mathematica

Mod[Prime[Range[100]], 21]

PARI

a(n) = prime(n) % 21; \\ Michel Marcus, May 05 2014

Sage

[mod(p, 21) for p in primes(500)] # Bruno Berselli, May 05 2014

Formula

Sum_k={1..n} a(k) ~ (21/2)*n. - Amiram Eldar, Dec 12 2024