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Ramanujan's ternary quadratic form : ウィキペディア英語版 | Ramanujan's ternary quadratic form In mathematics, in number theory, Ramanujan's ternary quadratic form is the algebraic expression ''x''2 + ''y''2 + 10''z''2 with integral values for ''x'', ''y'' and ''z''. Srinivasa Ramanujan considered this expression in a footnote in a paper published in 1916 and briefly discussed the representability of integers in this form. After giving necessary and sufficient conditions that an integer cannot be represented in the form ''ax''2 + ''by''2 + ''cz''2 for certain specific values of ''a'', ''b'' and ''c'', Ramanujan observed in a footnote: "(These) results may tempt us to suppose that there are similar simple results for the form ''ax''2 + ''by''2 + ''cz''2 whatever are the values of ''a'', ''b'' and ''c''. It appears, however, that in most cases there are no such simple results."〔 To substantiate this observation, Ramanujan discussed the form which is now referred to as Ramanujan's ternary quadratic form. ==Properties discovered by Ramanujan== In his 1916 paper〔 Ramanujan made the following observations about the form ''x''2 + ''y''2 + 10''z''2. *The even numbers which are not of the form ''x''2 + ''y''2 + 10''z''2 are 4λ(16μ + 6). *The odd numbers that are not of the form ''x''2 + ''y''2 + 10''z''2, viz. 3, 7, 21, 31, 33, 43, 67, 79, 87, 133, 217, 219, 223, 253, 307, 391, . . . do not seem to obey any simple law.
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