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Search: id:A045996
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| A045996 |
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Triangles in an n X n grid (or geoplane). |
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+0
3
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| 0, 4, 76, 516, 2148, 6768, 17600, 40120, 82608, 157252, 280988, 477012, 775172, 1214768, 1844512, 2725000, 3930384, 5550844, 7692300, 10482124, 14066996, 18619128, 24337056, 31449200, 40212160, 50921316, 63907468, 79542108
(list; graph; listen)
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OFFSET
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1,2
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FORMULA
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a(n) = ((n - 1)^2*n^2*(n + 1)^2)/6 - 2*Sum(Sum((n - k + 1)*(n - l + 1)*gcd(k - 1, l - 1), k, 2, n), l, 2, n).
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EXAMPLE
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a(2)=4 because 4 isosceles right triangles can be placed on a 2 X 2 grid.
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CROSSREFS
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Sequence in context: A067921 A101718 A094160 this_sequence A114453 A093184 A144210
Adjacent sequences: A045993 A045994 A045995 this_sequence A045997 A045998 A045999
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KEYWORD
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nice,nonn,easy
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AUTHOR
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Ignacio Larrosa Canestro (ignacio.larrosa(AT)eresmas.net)
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