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 A088817 Numbers k such that Cyclotomic(2k,k) is prime. 3
 1, 2, 3, 4, 5, 9, 17, 36, 157, 245, 352, 3977 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS This is a generalization of A056826. Note that (n^n+1)/(n+1) = cyclotomic(2n,n) when n is prime. These are probable primes for n > 352. No others < 4700. All terms of this sequence that are greater than 3 are congruent to 0 or 1 mod 4. In general, if k = s^2*t where t is squarefree and t == 2, 3 (mod 4), then Cyclotomic(2k,t*x^2) is the product of two polynomials. See the Wikipedia link below. - Jianing Song, Sep 25 2019 LINKS Eric Weisstein's World of Mathematics, Cyclotomic Polynomial Wikipedia, Aurifeuillean factorization MATHEMATICA Do[p=Prime[n]; If[PrimeQ[Cyclotomic[2n, n]], Print[p]], {n, 100}] PROG (PARI) is(n)=ispseudoprime(polcyclo(2*n, n)) \\ Charles R Greathouse IV, May 22 2017 CROSSREFS Cf. A056826 ((k^k+1)/(k+1) is prime), A070519 (cyclotomic(k,k) is prime), A088875 (cyclotomic(k,-k) is prime). Sequence in context: A049796 A106165 A305237 * A018896 A162374 A323289 Adjacent sequences:  A088814 A088815 A088816 * A088818 A088819 A088820 KEYWORD hard,nonn AUTHOR T. D. Noe, Oct 20 2003 STATUS approved

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Last modified April 10 08:09 EDT 2021. Contains 342845 sequences. (Running on oeis4.)