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 A067756 Prime hypotenuses of Pythagorean triangles with a prime leg. 13
 5, 13, 61, 181, 421, 1741, 1861, 2521, 3121, 5101, 8581, 9661, 16381, 19801, 36721, 60901, 71821, 83641, 100801, 106261, 135721, 161881, 163021, 199081, 205441, 218461, 273061, 282001, 337021, 388081, 431521, 491041, 531481, 539761, 552301 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS Apart from the first two terms, every term is congruent to 1 modulo 60 and is of the form 450k^2 +- 30k + 1 or 450k^2 +- 330k + 61 for some k. Every term of the sequence after the second is a prime p congruent to 1 (mod 60), i.e., for n>2, a(n) is a subsequence of A088955. The Pythagorean triple is {sqrt(2p-1), p-1, p}. - Lekraj Beedassy, Mar 12 2002 Primes p such that 2*p-1 is the square of a prime. - Robert Israel, Sep 16 2014 The offset probably should be 1, but this sequence is exceptional and has offset 0. Please do not change it! - N. J. A. Sloane, Sep 30 2014 Primes p of the form ((q+1)/2)^2 + ((q-1)/2)^2, where q is a prime; then q belongs to A048161. - Thomas Ordowski, May 22 2015 Other, larger, leg of Pythagorean triangle is p-1. - Zak Seidov, Oct 30 2015 LINKS Andreas Boe and Robert Israel, Table of n, a(n) for n = 0..10000 (0 to 183 from Andreas Boe). H. Dubner and T. Forbes, Prime Pythagorean triangles, J. Integer Seqs., Vol. 4 (2001), #01.2.3. FORMULA a(n) = (A048161(n)^2 + 1)/2 = A067755(n) + 1. EXAMPLE For p(1)=5, the right triangle 3, 4, 5 is the smallest with 3 and 5 prime. For p(10)=5101, the right triangle is 101, 5100, 5101 with 101 and 5101 prime. MAPLE N:= 10^8: # to get all terms <= N Primes:= select(isprime, [\$3..floor(sqrt(2*N-1))]): f:= proc(p) local q; q:= (p^2+1)/2; if isprime(q) then q else NULL fi end proc: map(f, Primes); # Robert Israel, Sep 16 2014 MATHEMATICA f[n_]:=((p-1)/2)^2+((p+1)/2)^2; lst={}; Do[p=Prime[n]; If[PrimeQ[f[p]], AppendTo[lst, f[p]]], {n, 6!}]; lst (* Vladimir Joseph Stephan Orlovsky, Jul 27 2009 *) PROG (PARI) forprime(p=3, 10^3, if(isprime(q=(p^2+1)/2), print1(q, ", "))) \\ Derek Orr, Apr 30 2015 CROSSREFS Contains every value of A051859. Sequence in context: A096639 A092773 A230444 * A284035 A051859 A151275 Adjacent sequences:  A067753 A067754 A067755 * A067757 A067758 A067759 KEYWORD nonn AUTHOR Henry Bottomley, Jan 31 2002 STATUS approved

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