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A137827 Prime powers (A246655) congruent to 1 (mod 3). 2
4, 7, 13, 16, 19, 25, 31, 37, 43, 49, 61, 64, 67, 73, 79, 97, 103, 109, 121, 127, 139, 151, 157, 163, 169, 181, 193, 199, 211, 223, 229, 241, 256, 271, 277, 283, 289, 307, 313, 331, 337, 343, 349, 361, 367, 373, 379, 397, 409, 421, 433, 439, 457, 463, 487, 499 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Numbers k not being powers of 3 such that x^2 + x + 1 (or x^2 - x + 1) is reducible over GF(k). - Jianing Song, Sep 24 2019

LINKS

Table of n, a(n) for n=1..56.

Eric Weisstein's World of Mathematics, Cyclotomic Graph

MATHEMATICA

Select[ Range[4, 500], (Mod[#, 3] == 1 && Mod[#, # - EulerPhi[#]] == 0)& ] (* Jean-Fran├žois Alcover, Oct 26 2012 *)

PROG

(PARI) isok(n) = isprimepower(n) && ((n % 3) == 1); \\ Michel Marcus, Sep 24 2019

CROSSREFS

Intersection of A016777 and A246655.

Sequence in context: A191201 A084089 A202822 * A045090 A213382 A258867

Adjacent sequences:  A137824 A137825 A137826 * A137828 A137829 A137830

KEYWORD

nonn,easy

AUTHOR

Eric W. Weisstein, Feb 12 2008

STATUS

approved

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Last modified November 15 11:18 EST 2019. Contains 329144 sequences. (Running on oeis4.)