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 A002479 Numbers of form x^2 + 2y^2. (Formerly M0547 N0197) 48
 0, 1, 2, 3, 4, 6, 8, 9, 11, 12, 16, 17, 18, 19, 22, 24, 25, 27, 32, 33, 34, 36, 38, 41, 43, 44, 48, 49, 50, 51, 54, 57, 59, 64, 66, 67, 68, 72, 73, 75, 76, 81, 82, 83, 86, 88, 89, 96, 97, 98, 99, 100, 102, 107, 108, 113, 114, 118, 121, 123, 128, 129, 131 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A positive number n belongs to this sequence if and only if every prime p == 5, 7 mod 8 dividing n occurs to an even power. - Sharon Sela (sharonsela(AT)hotmail.com), Mar 23 2002 Norms of numbers in Z[sqrt(-2)]. - Alonso del Arte, Sep 23 2014 Euler (E256) shows that these numbers are closed under multiplication, according to the Euler Archive. - Charles R Greathouse IV, Jun 16 2016 REFERENCES L. Euler, (E388) Vollstaendige Anleitung zur Algebra, Zweiter Theil, reprinted in: Opera Omnia. Teubner, Leipzig, 1911, Series (1), Vol. 1, p. 421. D. H. Lehmer, Guide to Tables in the Theory of Numbers. Bulletin No. 105, National Research Council, Washington, DC, 1941, p. 59. N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence). N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS T. D. Noe and N. J. A. Sloane, Table of n, a(n) for n = 1..3148 (First 1000 terms from T. D. Noe) L. Euler, (E256) Specimen de usu observationum in mathesi pura, Novi Commentarii academiae scientiarum Petropolitanae 6 (1761), pp. 185-230. N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references) MAPLE lis:={}; M:=50; M2:=M^2; for x from 0 to M do for y from 0 to M do if x^2+2*y^2 <= M2 then lis:={op(lis), x^2+2*y^2}; fi; od: od: sort(convert(lis, list)); # N. J. A. Sloane, Apr 30 2015 MATHEMATICA q = 16; imax = q^2; Select[Union[Flatten[Table[x^2 + 2y^2, {y, 0, q/Sqrt[2]}, {x, 0, q}]]], # <= imax &] (* Vladimir Joseph Stephan Orlovsky, Apr 20 2011 *) Union[#[[1]]+2#[[2]]&/@Tuples[Range[0, 10]^2, 2]] (* Harvey P. Dale, Nov 24 2014 *) PROG (PARI) is(n)=my(f=factor(n)); for(i=1, #f[, 1], if(f[i, 1]%8>4 && f[i, 2]%2, return(0))); 1 \\ Charles R Greathouse IV, Nov 20 2012 (PARI) list(lim)=my(v=List()); for(a=0, sqrtint(lim\=1), for(b=0, sqrtint((lim-a^2)\2), listput(v, a^2+2*b^2))); Set(v) \\ Charles R Greathouse IV, Jun 16 2016 (Haskell) a002479 n = a002479_list !! (n-1) a002479_list = 0 : filter f [1..] where    f x = all (even . snd) \$ filter ((`elem` [5, 7]) . (`mod` 8) . fst) \$                             zip (a027748_row x) (a124010_row x) -- Reinhard Zumkeller, Feb 20 2014 (MAGMA) [n: n in [0..131] | NormEquation(2, n) eq true]; // Arkadiusz Wesolowski, May 11 2016 CROSSREFS Complement of A097700. For primes see A033203. Cf. A035251, A027748, A124010, A003628. Sequence in context: A207432 A020900 A282697 * A010458 A218980 A084829 Adjacent sequences:  A002476 A002477 A002478 * A002480 A002481 A002482 KEYWORD easy,nonn,nice AUTHOR STATUS approved

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Last modified March 18 22:11 EDT 2019. Contains 321305 sequences. (Running on oeis4.)