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 A189412 Number of concave quadrilaterals on an n X n grid (or geoboard). 5
 0, 0, 24, 720, 6300, 34812, 135552, 436944, 1198968, 2929656, 6516984, 13502448, 26208516, 48407988, 85481280, 145200888, 238502808, 380729160, 591761304, 899049096, 1336994100, 1950873276, 2798226336, 3952174032, 5500597632, 7555866072, 10253438688 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 LINKS Michal ForiĊĦek, Table of n, a(n) for n = 1..50 Terr, David; Weisstein, Eric W.: MathWorld -- Concave Polygon. Weisstein, Eric W.: MathWorld -- Quadrilateral. PROG Python: def gcd(x, y): ..x, y = abs(x), abs(y) ..while y: x, y = y, x%y ..return x def concave(N): ..V = [ (r, c) for r in range(-N+1, N) for c in range(N) if (c>0 or r>0) ] ..answer = 0 ..for i in range(len(V)): ....for j in range(i): ......r1, c1, r2, c2 = V[i]+V[j] ......rr, cr, ta = N-max(r1, r2, 0)+min(r1, r2, 0), N-max(c1, c2), abs(r1*c2-r2*c1) ......if rr>0 and cr>0 and ta>0: ........answer += 3*rr*cr*(ta+2-gcd(r1, c1)-gcd(r2, c2)-gcd(r1-r2, c1-c2))/2 ..return answer CROSSREFS Cf. A175383, A189345, A189413, A189414. Sequence in context: A322746 A184274 A093456 * A246192 A246612 A289635 Adjacent sequences:  A189409 A189410 A189411 * A189413 A189414 A189415 KEYWORD nonn AUTHOR Martin Renner, Apr 21 2011 EXTENSIONS a(6) - a(22) from Nathaniel Johnston, Apr 25 2011 Terms a(7) - a(22) corrected by Michal Forisek, Sep 06 2011 Terms a(23) - a(50) added by Michal Forisek, Sep 06 2011 STATUS approved

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Last modified October 23 19:59 EDT 2019. Contains 328373 sequences. (Running on oeis4.)