%I A027861
%S A027861 1,2,4,5,7,9,12,14,17,19,22,24,25,29,30,32,34,35,39,42,47,50,60,
%T A027861 65,69,70,72,79,82,84,85,87,90,97,99,100,102,104,109,110,115,122,
%U A027861 130,135,137,139,144,149,154,157,160,162,164,167
%N A027861 Numbers n such that n^2 + (n+1)^2 is prime.
%C A027861 n>1 never ends in 1, 3, 6 or 8, (i.e. n*(n+1) does not end in 2). - Lekraj 
               Beedassy (blekraj(AT)yahoo.com), Jul 09 2004
%C A027861 n can never be congruent to (1 or 3) mod 5. Because if it were then n^2 
               + (n+1)^2 would be divisible by 5. In other words for n>1, this sequence 
               cannot contain any values in A047219. This means that we can immediately 
               discard 40% of all possible n. - Dmitry Kamenetsky (dkamen(AT)rsise.anu.edu.au), 
               Sep 02 2008
%H A027861 T. D. Noe, <a href="b027861.txt">Table of n, a(n) for n=1..1000</a>
%H A027861 P. De Geest, <a href="http://www.worldofnumbers.com/index.html">World!Of 
               Numbers</a>
%t A027861 lst={};Do[If[PrimeQ[n^2+(n+1)^2], Print[n];AppendTo[lst, n]], {n, 10^5}];
               lst [From Vladimir Orlovsky (4vladimir(AT)gmail.com), Aug 21 2008]
%Y A027861 Complement of A012132 - Michael Somos, Jun 08, 2000.
%Y A027861 Equals (A002731(n+1)-1)/2. A027862 gives primes, A091277 gives prime 
               index.
%Y A027861 Cf. A047219.
%Y A027861 Sequence in context: A158618 A000788 A053039 this_sequence A062428 A056833 
               A105771
%Y A027861 Adjacent sequences: A027858 A027859 A027860 this_sequence A027862 A027863 
               A027864
%K A027861 nonn,easy
%O A027861 1,2
%A A027861 Patrick De Geest (pdg(AT)worldofnumbers.com)