A098166 a(n) is the prime index of the Lesser Fortunate number A055211(n), or 0 if A055211(n) is not a prime.

2, 4, 5, 6, 7, 10, 9, 14, 13, 21, 17, 15, 24, 19, 21, 28, 24, 26, 31, 25, 23, 24, 25, 54, 32, 30, 36, 56, 54, 48, 41, 77, 60, 36, 45, 40, 52, 51, 43, 48, 48, 62, 108, 62, 88, 49, 64, 74, 55, 63, 65, 111, 69, 66, 63, 123, 91, 66, 83, 92, 97, 110, 67, 75, 99, 87

Offset: 2

Author

Pierre CAMI, Aug 30 2004

Keywords

• nonn

Links

• Jinyuan Wang, Table of n, a(n) for n = 2..2000
• Antonín Čejchan, Michal Křížek, and Lawrence Somer, On Remarkable Properties of Primes Near Factorials and Primorials, Journal of Integer Sequences, Vol. 25 (2022), Article 22.1.4.

Crossrefs

Cf. A000720, A055211, A098167.

Programs

PARI

a(n) = {my(k = prod(j=1, n, prime(j))); p = k-precprime(k-2); if(isprime(p), primepi(p), 0); } \\ Jinyuan Wang, Oct 02 2019

Formula

a(n) = A000720(A055211(n)) iff A055211(n) is a prime. - Michel Marcus, Oct 05 2019

Extensions

Name edited by and more terms from Jinyuan Wang, Oct 02 2019