A380091 Primes p such that phi(p+1) = 2*phi(p-1) where phi = A000010.

2, 3, 7, 31, 991, 1951, 2521, 7411, 23431, 26731, 37441, 92431, 131071, 396631, 489061, 532141, 830551, 2811691, 3319171, 3698941, 4247167, 5239411, 6829681, 8326711, 8997871, 12625831, 12889231, 14756743, 15891121, 16125721, 16446301, 21203071

Offset

1,1

Comments

Primes in A258454 - 1.

Links

Table of n, a(n) for n=1..32.

Mathematica

Select[Prime[Range[10^5]], EulerPhi[# + 1] == 2*EulerPhi[# - 1] &] (* Amiram Eldar, Jan 12 2025 *)

Prog

(Magma) [p: p in PrimesUpTo(15*10^6) | 2*EulerPhi(p-1) eq EulerPhi(p+1)];

Crossrefs

Cf. A000010, A066812, A067890, A258454.

Sequence in context: A018239 A096350 A066279 * A337221 A161471 A057677

Adjacent sequences: A380088 A380089 A380090 * A380092 A380093 A380094

Keyword

nonn

Author

Juri-Stepan Gerasimov, Jan 11 2025

Status

approved