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 A247979 Numbers of the form x^2 + 7y^2 with x, y integers, or x^2/4 + 7y^2/4 with x, y odd integers. 0
 0, 1, 2, 4, 7, 8, 9, 11, 14, 16, 18, 21, 22, 23, 25, 28, 29, 32, 36, 37, 43, 44, 46, 49, 50, 53, 56, 58, 63, 64, 67, 71, 72, 74, 77, 79, 81, 86, 88, 92, 98, 99, 100, 106, 107, 109, 112, 113, 116, 121, 126, 127, 128, 134, 137, 142, 144, 148, 149, 151, 154, 158, 161, 162, 163, 169, 172 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Norms of numbers in O_Q(sqrt(-7)). A033207 and A045386 are subsets of this sequence. - Colin Barker, Sep 29 2014 LINKS EXAMPLE 1/4 + 7/4 = 2, so 2 is in the sequence. (This also means 2 is composite in O_Q(sqrt(-7))). 2^2 + 7 * 0^2 = 4, so 4 is in the sequence. There is no way to express 5 as x^2 + 7y^2, nor as x^2/4 + 7y^2/4 if x and y are constrained to odd integers, hence 5 is not in the sequence. (This also means 5 is prime in O_Q(sqrt(-7)) and its norm is 25). CROSSREFS Cf. A002481, A033207, A045386. Sequence in context: A035261 A047351 A242937 * A028951 A035248 A190244 Adjacent sequences:  A247976 A247977 A247978 * A247980 A247981 A247982 KEYWORD easy,nonn AUTHOR Alonso del Arte, Sep 28 2014 STATUS approved

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Last modified June 19 03:23 EDT 2021. Contains 345125 sequences. (Running on oeis4.)