A121837 Least positive j such that Product_{k=1..n} D(k) + j is prime, where D() are the doublets, A020338.

2, 9, 7, 7, 17, 7, 29, 17, 19, 23, 23, 13, 29, 79, 19, 89, 97, 53, 43, 347, 127, 127, 149, 29, 167, 331, 379, 61, 59, 167, 199, 557, 107, 113, 43, 191, 439, 41, 263, 227, 109, 71, 227, 137, 149, 409, 271, 53, 157, 79, 503, 103, 461, 137, 587, 233, 491, 73, 367, 233, 449

Offset: 1

Author

Jason Earls, Aug 28 2006

Keywords

• nonn
• base

Comments

Is every term for n > 2 always prime?

a(159) = 1. - Michel Marcus, Jan 07 2021

a(n) = 1 for n = 245 and 702 (using ispseudoprime() in PARI). - Michel Marcus, Jan 08 2021

Links

• Michel Marcus, Table of n, a(n) for n = 1..200

Crossrefs

Cf. A013632, A020338, A121826, A151800.

Programs

PARI

D(n) = eval(Str(n, n)); \\ A020338
f(n) = prod(k=1, n, D(k)); \\ A121826
a(n) = my(q=f(n)); nextprime(q+1) - q; \\ Michel Marcus, Jan 07 2021

Formula

a(n) = A013632(A121826(n)). - Michel Marcus, Jan 07 2021