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
Keywords
Programs
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Magma
[p: p in PrimesUpTo(15*10^6) | 2*EulerPhi(p-1) eq EulerPhi(p+1)];
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Mathematica
Select[Prime[Range[10^5]], EulerPhi[# + 1] == 2*EulerPhi[# - 1] &] (* Amiram Eldar, Jan 12 2025 *)
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