The OEIS is supported by the many generous donors to the OEIS Foundation.
%I #9 Mar 15 2020 07:25:26
%S 216,660,1484,1560,8208,52164,544320,592956,649800,4321800,5103210,
%T 6182220,10621380,21415680,24471720,135307008,359624088,535019100,
%U 1071782250,1113233520,1227427740,1527496740,9462748008,143935711920
%N Numbers k such that k^2 + 1 is a Fermat pseudoprime to base 3.
%C a(24) > 7*10^10.
%C The corresponding pseudoprimes a(n)^2 + 1 are 46657, 435601, 2202257, 2433601, 67371265, ...
%C a(25) > 7.5*10^11. - _Giovanni Resta_, Mar 15 2020
%e 216 is a term since 216^2 + 1 = 46657 is a Fermat pseudoprime to base 3.
%t Select[Range[10^3], CompositeQ[#^2 + 1] && PowerMod[3, #^2, #^2 + 1] == 1 &]
%Y Cf. A002522, A005574, A005935, A135590.
%K nonn,more
%O 1,1
%A _Amiram Eldar_, Mar 14 2020
%E a(24) from _Giovanni Resta_, Mar 15 2020