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 A047221 Numbers that are congruent to {2, 3} mod 5. 36
 2, 3, 7, 8, 12, 13, 17, 18, 22, 23, 27, 28, 32, 33, 37, 38, 42, 43, 47, 48, 52, 53, 57, 58, 62, 63, 67, 68, 72, 73, 77, 78, 82, 83, 87, 88, 92, 93, 97, 98, 102, 103, 107, 108, 112, 113, 117, 118, 122, 123, 127, 128, 132, 133, 137, 138, 142, 143, 147, 148, 152, 153 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Theorem: if 5^((n-1)/2) = -1 (mod n) then n == 2 or 3 (mod 5) (see Crandall and Pomerance). Start with 2. The next number, 3, cannot be written as the sum of two of the previous terms. So 3 is in. 4=2+2, 5=2+3, 6=3+3, so these are not in. But you cannot obtain 7, so the next term is 7. And so on. - Fabian Rothelius, Mar 13 2001 Primitive roots of 5. The first differences are periodic: 1,4,1,4,1,4.... - Paolo P. Lava, Feb 29 2008 Also numbers n such that n^2 = -1 (mod 5). - Vincenzo Librandi, Aug 05 2010 For any (t,s)

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Last modified February 27 15:59 EST 2020. Contains 332307 sequences. (Running on oeis4.)