A375915 Composite numbers k == 1, 9 (mod 10) such that 5^((k-1)/2) == 1 (mod k).

781, 1541, 1729, 5461, 5611, 6601, 7449, 11041, 12801, 13021, 14981, 15751, 15841, 21361, 24211, 25351, 29539, 38081, 40501, 41041, 44801, 47641, 53971, 67921, 75361, 79381, 90241, 100651, 102311, 104721, 106201, 106561, 112141, 113201, 115921, 133141, 135201, 141361

Offset

1,1

Comments

Odd composite numbers k such that 5^((k-1)/2) == (5/k) = 1 (mod k), where (5/k) is the Jacobi symbol (or Kronecker symbol).

Links

Jianing Song, Table of n, a(n) for n = 1..1000

Example

29539 is a term because 29539 = 109*271 is composite, 29539 == 9 (mod 10), and 5^((29539-1)/2) == 1 (mod 29539).

Mathematica

q[k_] := MemberQ[{1, 9}, Mod[k, 10]] && CompositeQ[k] && PowerMod[5, (k-1)/2, k] == 1; Select[Range[142000], q] (* Amiram Eldar, Mar 21 2026 *)

Prog

(PARI) isA375915(k) = (k>1) && !isprime(k) && (k%10==1 || k%10==9) && Mod(5, k)^((k-1)/2) == 1

Crossrefs

For a list of sequences related to Euler-Jacobi pseudoprimes and Euler pseudoprimes, see A306310.

Sequence in context: A115467 A338877 A375914 * A020231 A141390 A038477

Adjacent sequences: A375912 A375913 A375914 * A375916 A375917 A375918

Keyword

nonn

Author

Jianing Song, Sep 02 2024

Status

approved