A060257 Numbers k such that 1/prime(k) has period prime(k) - 1.

4, 7, 8, 9, 10, 15, 17, 18, 25, 29, 30, 32, 35, 39, 41, 42, 44, 48, 50, 51, 55, 56, 57, 65, 68, 73, 75, 76, 77, 81, 84, 89, 93, 94, 95, 96, 97, 100, 105, 106, 108, 114, 118, 120, 126, 127, 129, 132, 141, 142, 143, 148, 150, 154, 159, 160, 162, 164, 165, 166, 171

Offset: 1

Author

Jeff Burch, Mar 22 2001

Keywords

• nonn

Links

• Amiram Eldar, Table of n, a(n) for n = 1..10000
• Matt Parker and Brady Haran, The Reciprocals of Primes, Numberphile video (2022)

Crossrefs

Cf. A060258.

Cf. A000720 (PrimePi), A001913 (primes with primitive root 10).

Programs

Mathematica

Select[Range[200], MultiplicativeOrder[10, (p = Prime[#])] == p - 1 &] (* Amiram Eldar, Oct 03 2021 *)

Formula

a(n) = PrimePi(A001913(n)). - Alexander Adamchuk, Jan 28 2007