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 A160054 Primes prime(k) such that prime(k)^2 + prime(k+1)^2 - 1 is a perfect square. 1
 7, 11, 23, 109, 211, 307, 1021, 4583, 42967, 297779, 1022443, 1459811, 10781809, 125211211, 11673806759, 3019843939831 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS An infinite number of solutions exists for a^2 + b^2 - 1 = c^2 over the set of natural numbers a, b, c. If we constrain these to b=a+2, i.e., 2a^2 + 4a + 3 = c^2, the solutions are with a = 1, 11, 69, 407, 2377, ... (The twin prime 11 is also in this sequence here. The solutions can be generated recursively from a(0)=1, m(0)=3 and a(k+1) = 3*a(k) + 2*m(k) + 2, m(k+1) = 4*a(k) + 3*m(k) + 4.) Filtering these solutions for prime pairs a(k) and b(k) would generate the subset of lower twin primes in the sequence. The equivalent procedure can be carried out for other prime gaps 2*d such that prime(k)=a, prime(k+1)=a+2*d, 2*a^2 + 4*a*d + 4*d^2 - 1 = m^2. This decomposes the sequence into classes according to the gap 2*d. a(17) > 5*10^12. - Donovan Johnson, May 17 2010 LINKS FORMULA {A000040(k): A069484(k)-1 in A000290}. EXAMPLE 7^2+11^2-1=13^2. 11^2+13^2-1=17^2. 23^2+29^2-1=37^2. 109^2+113^2-1=157^2. 211^2+223^2-1=307^2. 307^2+311^2-1=19^2*23^2. 1021^2+1031^2-1=1451^2. 4583^2+4591^2-1=13^2*499^2. MATHEMATICA lst = {}; p = q = 2; While[p < 4000000000, q = NextPrime@ p; If[ IntegerQ[ Sqrt[p^2 + q^2 - 1]], AppendTo[lst, p]; Print@ p]; p = q]; lst (* Robert G. Wilson v, May 31 2009 *) PROG (PARI) p=2; forprime(q=3, 1e6, if(issquare(q^2+p^2-1), print1(p", ")); p=q) \\ Charles R Greathouse IV, Nov 06 2014 (PARI) is(n)=issquare(n^2+nextprime(n+1)^2-1)&&isprime(n) \\ Charles R Greathouse IV, Nov 29 2014 (MAGMA) [n: n in [0..2*10^7] | IsSquare(n^2+NextPrime(n+1)^2-1) and IsPrime(n)]; // Vincenzo Librandi, Aug 02 2015 CROSSREFS Cf. A129288, A050791. Sequence in context: A243916 A181841 A076855 * A247590 A027830 A134043 Adjacent sequences:  A160051 A160052 A160053 * A160055 A160056 A160057 KEYWORD nonn,more AUTHOR Ulrich Krug (leuchtfeuer37(AT)gmx.de), May 01 2009 EXTENSIONS Edited and 4 more terms from R. J. Mathar, May 08 2009 a(13) from Robert G. Wilson v, May 31 2009 a(15)-a(16) from Donovan Johnson, May 17 2010 STATUS approved

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Last modified July 14 09:17 EDT 2020. Contains 335720 sequences. (Running on oeis4.)