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 A090698 Primes of the form 2*n^2+1. 13
 3, 19, 73, 163, 883, 1153, 1459, 1801, 2179, 2593, 3529, 4051, 8713, 10369, 11251, 15139, 17299, 18433, 19603, 20809, 22051, 30259, 34849, 36451, 46819, 48673, 52489, 62659, 69193, 71443, 80803, 83233, 95923, 103969, 112339, 115201, 130051 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A prime p can be expressed as either the sum of two squares or the sum of two squares - 1, p = X^2 + Y^2 or p = X^2 + Y^2 - 1, if and only if p is of the form 2*(m^2)+1 where m is either 1 or a multiple of 3. Conjecture: 2^(a(n)-1) - 3 is not prime. - Vincenzo Librandi, Feb 04 2013. Primes in A058331. - Vincenzo Librandi, Apr 10 2015 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..5000 FORMULA a(n)=2*A089001(n)^2+1 = A000040(A090612(n)). EXAMPLE 19 = 2^2 + 4^2 - 1 = 2*(3^2)+1 73 = 5^2 + 7^2 - 1 = 2*(6^2)+1 163= 8^2 + 10^2 -1 = 2*(9^2)+1 883= 10^2+ 28^2 -1 = 2*(21^2)+1 MATHEMATICA Select[Table[2n^2+1, {n, 0, 900}], PrimeQ] (* Vincenzo Librandi, Dec 02 2011 *) PROG (MAGMA)[a: n in [0..400] | IsPrime(a) where a is 2*n^2+1]; // Vincenzo Librandi, Dec 02 2011 (PARI) is(n)=isprime(2*n^2+1) \\ Charles R Greathouse IV, Jan 05 2013 CROSSREFS Cf. A058331, A089001, A089008, A090612. Sequence in context: A059599 A183461 A095662 * A350713 A215802 A202041 Adjacent sequences:  A090695 A090696 A090697 * A090699 A090700 A090701 KEYWORD nonn,easy AUTHOR Kurmang. Aziz. Rashid, Dec 20 2003 EXTENSIONS Extended by Ray Chandler, Dec 21 2003 STATUS approved

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Last modified July 5 05:10 EDT 2022. Contains 355087 sequences. (Running on oeis4.)