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A189416 Number of parallelograms on an n X n grid. 4
0, 1, 22, 158, 674, 2159, 5664, 13004, 26904, 51401, 92094, 156710, 255090, 400359, 608656, 900100, 1299336, 1836461, 2546550, 3472162, 4661898, 6173123, 8071952, 10434600, 13346080, 16905033, 21221558, 26419338, 32636098, 40027283, 48761448 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

LINKS

Table of n, a(n) for n=1..31.

Nathaniel Johnston, C program for computing terms

Eric Weisstein's World of Mathematics, Parallelogram

FORMULA

a(n) = Sum_{a=1..n-1} Sum_{b=1..n-1} (n-a)*(n-b)*(2*a*b - gcd(a,b)). - Andrew Howroyd, Sep 19 2017

MATHEMATICA

a[n_] := Sum[(n-a)*(n-b)*(2*a*b - GCD[a, b]), {a, 1, n-1}, {b, 1, n-1}];

Array[a, 31] (* Jean-Fran├žois Alcover, Oct 08 2017, translated from PARI *)

PROG

(PARI) a(n) = sum(a=1, n-1, sum(b=1, n-1, (n-a)*(n-b)*(2*a*b - gcd(a, b)) )); \\ Andrew Howroyd, Sep 19 2017

CROSSREFS

Cf. A186434, A189413, A189414.

Sequence in context: A224257 A244868 A223913 * A224185 A185859 A173393

Adjacent sequences:  A189413 A189414 A189415 * A189417 A189418 A189419

KEYWORD

nonn

AUTHOR

Martin Renner, Apr 21 2011

EXTENSIONS

a(6)-a(31) from Nathaniel Johnston, Apr 24 2011

STATUS

approved

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Last modified November 21 03:19 EST 2018. Contains 317427 sequences. (Running on oeis4.)