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 A204185 Number of quadrilaterals in a triangular matchstick arrangement of side n. 1
 0, 0, 6, 33, 102, 243, 492, 894, 1500, 2370, 3570, 5175, 7266, 9933, 13272, 17388, 22392, 28404, 35550, 43965, 53790, 65175, 78276, 93258, 110292, 129558, 151242, 175539, 202650, 232785, 266160, 303000, 343536, 388008, 436662, 489753, 547542, 610299, 678300, 751830, 831180, 916650, 1008546, 1107183, 1212882, 1325973, 1446792, 1575684, 1713000, 1859100, 2014350 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS The total number of parallelograms and trapezoids that appear in a triangular matchstick array of side n. Can always be split into three equal sets, parallelograms 'belonging' to the side of the triangle that none of its sides are parallel to, and trapezoids 'belonging' to the side of the triangle that two of its sides are parallel to. Rhombuses belonging to each side are A173196(n). Irregular parallelograms belonging to each side are 2(A001752(n-3)). 'Upside down' trapezoids (those where the shorter of the two parallel sides is closest to the parallel side of the triangle) belonging to each side are A001752(n-3). 'Right side up' trapezoids belonging to each side are A000332(n+2). LINKS Harvey P. Dale, Table of n, a(n) for n = 0..1000 Index entries for linear recurrences with constant coefficients, signature (4,-5,0,5,-4,1). FORMULA a(n) = b(1)T(n-1) + b(2)T(n-2) + b(3)T(n-3) ... + b(k)T(n-k) ... + b(n-1)T(1), where b(m) = 3(floor(5m/2)) and T(m) is the m-th triangular number A000217. a(n) = a(n-1) + floor((n+1)(n-1)(10n-3)/8). a(n) = 3(A173196(n)+A000332(n+2)+3(A001752(n-3)) (see comments above). From Colin Barker, Mar 16 2015: (Start) a(n) = (3-3*(-1)^n-16*n-16*n^2+16*n^3+10*n^4)/32. a(n) = 4*a(n-1)-5*a(n-2)+5*a(n-4)-4*a(n-5)+a(n-6). G.f.: -3*x^2*(3*x+2) / ((x-1)^5*(x+1)). (End) EXAMPLE a(3) = 33 because the following figure contains 33 quadrilaterals (15 parallelograms and 18 trapezoids) ....... /\ ...... /\/\ ..... /\/\/\ Size and quantity of each quadrilateral in above figure: 2 triangles: 9 3 triangles: 12 4 triangles: 6 5 triangles: 3 8 triangles: 3 MATHEMATICA nxt[{n_, a_}]:={n+1, a+Floor[n(n+2) (10(n+1)-3)/8]}; Transpose[ NestList[ nxt, {0, 0}, 50]][] (* Harvey P. Dale, Jan 11 2013 *) PROG (PARI) concat([0, 0], Vec(-3*x^2*(3*x+2)/((x-1)^5*(x+1)) + O(x^100))) \\ Colin Barker, Mar 16 2015 CROSSREFS Cf. A173196 = number of rhombuses of a particular orientation; A001752, related to number of irregular parallelograms and number of 'upside down' trapezoids; A000332, related to number of 'right side up' trapezoids (see comments above); A002717 = number of triangles in a triangular matchstick arrangement; A000217 = triangular numbers. Sequence in context: A153127 A274218 A135526 * A057818 A063267 A082106 Adjacent sequences:  A204182 A204183 A204184 * A204186 A204187 A204188 KEYWORD nonn,nice,easy AUTHOR Elliott Line & Paul Bostock (enigma.mensa(AT)yahoo.co.uk), Jan 12 2012 STATUS approved

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Last modified May 23 13:32 EDT 2019. Contains 323514 sequences. (Running on oeis4.)