# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a321868 Showing 1-1 of 1 %I A321868 #11 Jul 25 2019 06:46:23 %S A321868 341,645,2465,2821,4033,5461,8321,15841,25761,31621,68101,83333, %T A321868 162401,219781,282133,348161,530881,587861,653333,710533,722261, %U A321868 997633,1053761,1082401,1193221,1246785,1333333,1357441,1398101,1489665,1584133,1690501,1735841 %N A321868 Fermat pseudoprimes to base 2 that are octagonal. %C A321868 Rotkiewicz proved that under Schinzel's Hypothesis H this sequence is infinite. %C A321868 Intersection of A001567 and A000567. %C A321868 The corresponding indices of the octagonal numbers are 11, 15, 29, 31, 37, 43, 53, 73, 93, 103, 151, 167, 233, 271, 307, 341, 421, 443, 467, 487, 491, 577, 593, 601, 631, 645, 667, 673, 683, 705, 727, 751, 761, 901, 911, 919, 991, ... %C A321868 First differs from A216170 at n = 505. %H A321868 Amiram Eldar, Table of n, a(n) for n = 1..10000 %H A321868 Andrzej Rotkiewicz, On some problems of W. Sierpinski, Acta Arithmetica, Vol. 21 (1972), pp. 251-259. %H A321868 Wikipedia, Schinzel's Hypothesis H. %t A321868 oct[n_]:=n(3n-2); Select[oct[Range[1, 1000]], PowerMod[2, (# - 1), #]==1 &] %o A321868 (PARI) isok(n) = (n>1) && ispolygonal(n, 8) && !isprime(n) && (Mod(2, n)^n==2); \\ _Daniel Suteu_, Nov 29 2018 %Y A321868 Cf. A000567, A001567, A216170, A293623, A293624, A321869. %K A321868 nonn %O A321868 1,1 %A A321868 _Amiram Eldar_, Nov 20 2018 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE