A371760 a(n) is the smallest number k such that the k-th n-gonal number is a Fermat pseudoprime to base 2 (A001567), or -1 if no such number exists.
33, 1093, 73, 17, 97, 11, 193, 17, 89, 11, 193, 73, 673, 13, 257, 33, 41, 15, 97, 65, 1009, 13, 97, 149, 190, 23, 401, 41, 281, 31, 133, 17, 1033, 31, 89, 13, 6, 59, 241, 157, 1217, 91, 145, 37, 937, 29, 1289, 73, 97, 41, 617, 19, 137, 151, 34, 103, 8641, 47, 82
Offset: 3
Keywords
Links
- Amiram Eldar, Table of n, a(n) for n = 3..10000
- Eric Weisstein's World of Mathematics, Polygonal Number.
- Eric Weisstein's World of Mathematics, Poulet Number.
- Wikipedia, Polygonal number.
- Wikipedia, Pseudoprime.
- Index entries for sequences related to pseudoprimes.
Programs
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Mathematica
p[k_, n_] := ((n - 2)*k^2 - (n - 4)*k)/2; pspQ[n_] := CompositeQ[n] && PowerMod[2, n - 1, n] == 1; a[n_] := Module[{k = 2}, While[! pspQ[p[k, n]], k++]; k]; Array[a, 100, 3] -
PARI
p(k, n) = ((n-2)*k^2 - (n-4)*k)/2; ispsp(n) = !isprime(n) && Mod(2, n)^(n-1) == 1; a(n) = {my(k = 2); while(!ispsp(p(k, n)), k++); k;}
Comments