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URL: https://oeis.org/A065428

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A065428
Numbers k such that no x^2 mod k is prime.
6
1, 2, 3, 4, 5, 8, 12, 15, 16, 24, 28, 40, 48, 56, 60, 72, 88, 112, 120, 168, 232, 240, 280, 312, 408, 520, 760, 840, 1320, 1848
OFFSET
1,2
COMMENTS
All numbers in this sequence except 56 are idoneal (A000926) - Joerg Arndt, Jul 13 2005
No more terms < 10^6. - T. D. Noe, Aug 10 2007
No more terms < 10^11. - Charles R Greathouse IV, Dec 15 2008
Numbers x such that all x^3 mod k are nonprimes are 1, 2, 7, 9, 63, and apparently no more.
MATHEMATICA
t={}; Do[s=Union[Mod[Range[n]^2, n]]; If[Select[s, PrimeQ]=={}, AppendTo[t, n]], {n, 1000}]; t (* T. D. Noe, Aug 10 2007 *)
nx2pQ[n_]:=Module[{m=PowerMod[Range[3n], 2, n]}, Count[ FindTransientRepeat[ m, 2][[2]], _?PrimeQ]==0]; Select[Range[2000], nx2pQ] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jun 11 2019 *)
PROG
(PARI) for(n=1, 10^9, q=1; for(x=1, n-1, if(isprime(lift(Mod(x, n)^2)), q=0; break())); if(q, print1(n, ", "))); \\ edited, Joerg Arndt, Jan 28 2015
(Haskell)
a065428 n = a065428_list !! (n-1)
a065428_list = filter f [1..] where
f x = all (== 0) $
map (a010051' . (`mod` x) . a000290) [a000196 x .. x-1]
-- Reinhard Zumkeller, Aug 01 2012, Aug 15 2011
(Python)
from sympy import isprime
def ok(n): return not any(isprime((x**2)%n) for x in range(2, n))
print(list(filter(ok, range(1, 2000)))) # Michael S. Branicky, May 08 2021
CROSSREFS
Cf. A179402 (x^4 mod n).
Cf. A214583 (n such that for all k with gcd(n, k) = 1 and n > k^2, n - k^2 is prime).
Sequence in context: A122700 A048486 A179402 * A059747 A254328 A094087
KEYWORD
nonn,nice,hard,more
AUTHOR
Joerg Arndt, Nov 16 2001
STATUS
approved