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 A273529 Primes p of the form x^2 + y^2 such that p+1 is the sum of the two nonzero squares in exactly 2 ways. 1
 337, 409, 449, 577, 1009, 1129, 1489, 1801, 2377, 2521, 2609, 2689, 2833, 3041, 3169, 3329, 3361, 3433, 3529, 3889, 4049, 4513, 4657, 5209, 5569, 5689, 5857, 5881, 5953, 6529, 6553, 6569, 7177, 7297, 8009, 8089, 8209, 8329, 8641, 8737, 8761, 9433, 9697, 9769, 10169, 10321 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Number of prime divisors (counted with multiplicity) of p+1 is 3, 3, 5, 3, 3, 3, 3, 3, 3, 3, 5, 3, 3, 5, 3, 5, 3, 3, 3, 3, 7, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 5, 3, 3, 5, 3, 3, 5, 3, 3, 3, 3, 3, 3, 5, 3, 7, 3, 3, 5, 3, 3, 3, 5, 5, 3, 3, 3, 3, 3, ... In this sequence, 20249 is the first p such that p+1 has even number of prime divisors (counted with multiplicity). LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 FORMULA a(n) mod 8 = 1. EXAMPLE The prime 409 is a term because 409 = 3^2 + 20^2 and 410 = 7^2 + 19^2 = 11^2 + 17^2. PROG (PARI) is(n, k)=nb = 0; lim = sqrtint(n); for (x=1, lim, if ((n-x^2 >= x^2) && issquare(n-x^2), nb++); ); nb == k; isok(n) = isprime(n) && is(n, 1) && is(n+1, 2); (PARI) is(n)=if(n%8!=1 || !isprime(n), return(0)); my(f=factor((n+1)/2), t=1); for(i=1, #f~, if(f[i, 1]%4==1, t*=f[i, 2]+1, if(f[i, 2]%2, return(0)))); t==3 || t==4 \\ Charles R Greathouse IV, May 24 2016 CROSSREFS Cf. A002313, A025285. Sequence in context: A068298 A003790 A003924 * A153164 A020358 A260540 Adjacent sequences:  A273526 A273527 A273528 * A273530 A273531 A273532 KEYWORD nonn AUTHOR Altug Alkan, May 24 2016 STATUS approved

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Last modified April 21 23:52 EDT 2021. Contains 343156 sequences. (Running on oeis4.)