This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A122784 #16 Jul 24 2021 05:40:48
%S A122784 1,6,14,21,25,42,105,133,231,301,325,525,561,703,817,1105,1729,1825,
%T A122784 2101,2353,2465,2821,3277,3325,3486,3913,4011,4525,4825,5565,5719,
%U A122784 5901,6601,6697,7525,8321,8911,9331,10225,10325,10585,10621,11041,11521
%N A122784 Nonprimes k such that 7^k == 7 (mod k).
%C A122784 Theorem: If both numbers q and 2q-1 are primes then q*(2q-1) is in the sequence iff q=2 or mod(q,14) is in the set {1, 5, 13}. 6, 703, 18721, 38503, 88831, 104653, 146611, 188191,... are such terms.
%H A122784 Amiram Eldar, <a href="/A122784/b122784.txt">Table of n, a(n) for n = 1..10000</a>
%t A122784 Select[Range[20000], ! PrimeQ[#] && PowerMod[7, #, #] == Mod[7, #] &]
%t A122784 With[{nn=12000},Select[Complement[Range[nn],Prime[Range[PrimePi[ nn]]]], PowerMod[7,#,#]==Mod[7,#]&]] (* _Harvey P. Dale_, Jul 12 2012 *)
%Y A122784 Cf. A005938, A121014, A122780, A122781, A122782, A122783, A122785, A122786.
%K A122784 nonn
%O A122784 1,2
%A A122784 _Farideh Firoozbakht_, Sep 12 2006