%I #25 Feb 16 2025 08:33:06

%S 6,18,906,5466,257302,825366,1880082,6637546,6765826,8936722,9483706,

%T 34087054,51914026,54806454,57663334,57819882,67372378,91835206,

%U 98963734,102985926,117697186,134457346,143888806,172530646,206623266

%N Even pseudoprimes to base 19.

%H Amiram Eldar, <a href="/A130437/b130437.txt">Table of n, a(n) for n = 1..145</a> (terms below 10^11; terms 1..51 from Robert G. Wilson v)

%H Eric Weisstein's World of Mathematics, <a href="https://mathworld.wolfram.com/FermatPseudoprime.html">Fermat Pseudoprime</a>.

%H <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>.

%t lst = {}; Do[ If[ PowerMod[19, 2n - 1, 2n] == 1, AppendTo[lst, 2n]], {n, 2, 2*10^9}]; lst (* _Robert G. Wilson v_, Jun 01 2007 *)

%o (PARI) is(k) = k > 2 && !(k % 2) && Mod(19, k)^(k-1) == 1; \\ _Amiram Eldar_, Sep 29 2024

%Y Cf. A020147 = Pseudoprimes to base 19. Cf. A006935 = Even pseudoprimes (or primes) to base 2: n divides 2^n - 2, n even. Cf. A130433 = Even pseudoprimes to base 3. Cf. A090082 = Even pseudoprimes to base 5. Cf. A090083, A090084, A090085. Cf. A130434, A130435, A130436, A130438, A130439, A130440, A130441, A130442, A130443.

%Y Cf. A020147, A006935, A130433, A090082, A090083, A090084, A090085, A130434, A130435, A130436, A130438, A130439, A130440, A130441, A130442, A130443.

%K nonn

%O 1,1

%A _Alexander Adamchuk_, May 26 2007

%E More terms from _Robert G. Wilson v_, Jun 01 2007