A252021 Primes p such that (p - 1)/tau(p - 1) is also prime.

13, 19, 41, 61, 89, 137, 157, 229, 233, 277, 349, 373, 569, 709, 733, 809, 853, 857, 877, 997, 1049, 1069, 1097, 1193, 1213, 1237, 1433, 1669, 1789, 1913, 2153, 2293, 2389, 2677, 2749, 2777, 2797, 3209, 3229, 3253, 3373, 3449, 3517, 3593, 3733, 3833, 3929, 4073, 4457, 4549, 4597, 4793, 4813, 4909, 4937, 5197, 5273

Offset: 1

Author

Carlos Eduardo Olivieri, Mar 21 2015

Keywords

• nonn
• easy

Examples

			a(1) = 13, since 12/tau(12) = 2.
a(2) = 19, since 18/tau(18) = 3.
a(4) = 61, since 60/tau(60) = 5.
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A000005, A008328, A033950.

Programs

Magma

[p:p in PrimesUpTo(5300)| ((p-1) mod NumberOfDivisors(p-1) eq 0) and IsPrime((p-1) div NumberOfDivisors(p-1)) ]; // Marius A. Burtea, Dec 30 2019

Mathematica

Select[Prime[Range[1000]], PrimeQ[(# - 1)/DivisorSigma[0, # - 1]] &]