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A122781
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Nonprimes n such that 4^n==4 (mod n).
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9
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1, 4, 6, 12, 15, 28, 66, 85, 91, 186, 276, 341, 435, 451, 532, 561, 645, 703, 946, 1068, 1105, 1247, 1271, 1387, 1581, 1695, 1729, 1891, 1905, 2044, 2046, 2047, 2071, 2465, 2701, 2821, 2926, 3133, 3277, 3367, 3683, 4033, 4369, 4371, 4681, 4795
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OFFSET
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1,2
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COMMENTS
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If both numbers q and 2q-1 are prime, then q*(2q-1) is in the sequence. So, A005382(n)*(2*A005382(n)-1) = A129521(n) form a subsequence.
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LINKS
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MAPLE
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for n from 1 to 5000 do if 4^n mod n = 4 mod n and not isprime(n) then print(n) fi od; # Gary Detlefs, May 14 2012
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MATHEMATICA
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Select[Range[4800], ! PrimeQ[ # ] && Mod[4^#, # ] == Mod[4, # ] &]
Join[{1, 4}, Select[Range[5000], !PrimeQ[#]&&PowerMod[4, #, #]==4&]] (* Harvey P. Dale, Apr 09 2018 *)
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CROSSREFS
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KEYWORD
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easy,nonn
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AUTHOR
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STATUS
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approved
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