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 A038872 Primes congruent to {0, 1, 4} mod 5. 75
 5, 11, 19, 29, 31, 41, 59, 61, 71, 79, 89, 101, 109, 131, 139, 149, 151, 179, 181, 191, 199, 211, 229, 239, 241, 251, 269, 271, 281, 311, 331, 349, 359, 379, 389, 401, 409, 419, 421, 431, 439, 449, 461, 479, 491, 499, 509, 521, 541, 569, 571, 599, 601, 619 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Also odd primes p such that 5 is a square mod p: (5/p) = +1 for p > 5. Primes of the form x^2 + x*y - y^2 (as well as of the form x^2 + 3*x*y + y^2), both with discriminant = 5 and class number = 1. Binary quadratic forms a*x^2 + b*x*y + c*y^2 have discriminant d = b^2 - 4ac and gcd(a, b, c) = 1. [This was originally a separate entry, submitted by Laura Caballero Fernandez, Lourdes Calvo Moguer, Maria Josefa Cano Marquez, Oscar Jesus Falcon Ganfornina and Sergio Garrido Morales, Jun 06 2008. R. J. Mathar proved on Jul 22 2008 that this coincides with the present sequence.] Also primes of the form 5x^2 - y^2 (cf. A031363). - N. J. A. Sloane, May 30 2014 Also primes of the form x^2 + 4*x*y - y^2. Every binary quadratic primitive form of discriminant 20 or 5 has proper solutions for positive integers N given in A089270, including the present primes. Proof from computing the corresponding representative parallel primitive forms, which leads to x^2 - 5 == 0 (mod N) or x^2 + x - 1 == 0 (mod N) which have solutions precisely for these positive N values, including these primes. - Wolfdieter Lang, Jun 19 2019 For a Pythagorean triple a, b, c, these primes (and 2) are the possible prime factors of 2a + b, |2a - b|, 2b + a, and 2b - a. - J. Lowell, Nov 05 2011 The prime factors of A028387(n^2+3n+1). - Richard R. Forberg, Dec 12 2014 a(n) = A045468(n-1) for n > 1. - Robert Israel, Dec 22 2014 Except for p = 5, these are primes p that divide Fibonacci(p-1). - Dmitry Kamenetsky, Jul 27 2015 Apart from the first term, these are rational primes that decompose in the field Q[sqrt(5)]. For example, 11 = ((7 + sqrt(5))/2)*((7 - sqrt(5))/2), 19 = ((9 + sqrt(5))/2)*((9 - sqrt(5))/2). - Jianing Song, Nov 23 2018 The possible prime factors of x^2 - x - 1. - Charles R Greathouse IV, Mar 18 2022 REFERENCES Z. I. Borevich and I. R. Shafarevich, Number Theory. LINKS T. D. Noe, Table of n, a(n) for n = 1..1000 C. Banderier, Calcul de (5/p) Henri Darmon, Andrew Wiles’s Marvelous Proof, Notices of the AMS (2017), Volume 64, Number 3 pp. 209-216. See p. 211. Tamara M. Lavshuk, Regular polygons and polyhedra over finite field, Mathematical Notes of NEFU, Vol 22 No 4 (2015). Mentions this sequence. N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references) D. B. Zagier, Zetafunktionen und quadratische Körper, Springer, 1981. FORMULA a(n) ~ 2n*log(n). - Charles R Greathouse IV, Nov 29 2016 MAPLE select(isprime, [5, seq(op([5*k-1, 5*k+1]), k=1..1000)]); # Robert Israel, Dec 22 2014 MATHEMATICA Join[{5}, Select[Prime[Range[4, 100]], Mod[#, 5] == 1 || Mod[#, 5] == 4 &]] (* Alonso del Arte, Nov 27 2011 *) PROG (PARI) forprime(p=2, 1e3, if(kronecker(5, p)>=0, print1(p", "))) \\ Charles R Greathouse IV, Jun 16 2011 (Magma) [ p: p in PrimesUpTo(700) | p mod 5 in {0, 1, 4}]; // Vincenzo Librandi, Aug 21 2012 (GAP) Filtered(Concatenation([5], Flat(List([1..140], k->[5*k-1, 5*k+1]))), IsPrime); # Muniru A Asiru, Nov 24 2018 CROSSREFS Cf. A045468, A089270. Cf. A038872 (d=5); A038873 (d=8); A068228, A141123 (d=12); A038883 (d=13). A038889 (d=17); A141111, A141112 (d=65). Cf. A003631 (complement with respect to A000040). Sequence in context: A132087 A089270 A275068 * A141158 A239732 A130828 Adjacent sequences: A038869 A038870 A038871 * A038873 A038874 A038875 KEYWORD nonn,easy AUTHOR EXTENSIONS Corrected and extended by Peter K. Pearson, May 29 2005 Edited by N. J. A. Sloane, Jul 28 2008 at the suggestion of R. J. Mathar STATUS approved

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Last modified December 10 02:09 EST 2022. Contains 358712 sequences. (Running on oeis4.)