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A295321 Sum of the products of the smaller and larger parts of the partitions of n into two distinct parts with the larger part even. 2
0, 0, 2, 0, 4, 8, 18, 12, 26, 40, 64, 52, 82, 112, 156, 136, 188, 240, 310, 280, 360, 440, 542, 500, 614, 728, 868, 812, 966, 1120, 1304, 1232, 1432, 1632, 1866, 1776, 2028, 2280, 2570, 2460, 2770, 3080, 3432, 3300, 3674, 4048, 4468, 4312, 4756, 5200, 5694 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Sum of the areas of the distinct rectangles with even length and integer width such that L + W = n, W < L. For example, a(12) = 52; the rectangles are 2 X 10 and 4 X 8 (6 X 6 is not included since we have W < L), so 2*10 + 4*8 = 52.

Sum of the ordinates from the ordered pairs (n-k,n*k-k^2) corresponding to integer points along the right side of the parabola b_k = n*k-k^2 where n-k is an even integer such that 0 < k < floor(n/2).

Sum of the areas of the trapezoids with bases n and n-2i and height i for even values of n-i where i is in 0 <= i <= floor((n-1)/2). For a(n) the area formula for a trapezoid becomes (n+n-2i)*i/2 = (2n-2i)*i/2 = i*(n-i). For n=10, n-i is even when i=0,2,4 so a(10) = 0*(10-0) + 2*(10-2) + 4*(10-4) = 0 + 16 + 24 = 40. - Wesley Ivan Hurt, Mar 22 2018

Sum of the areas of the symmetric L-shaped polygons with long side n/2 and width i such that n-i is even for i in 0 <= i <= floor((n-1)/2). The area of each polygon is given by i^2+2i(n/2-i) = i^2+ni-2i^2 = i(n-i). For n=9, 9-i is even for i=1,3 so 1(9-1) + 3(9-3) = 8 + 18 = 26. - Wesley Ivan Hurt, Mar 26 2018

LINKS

Vincenzo Librandi, Table of n, a(n) for n = 1..5000

Index entries for sequences related to partitions

FORMULA

a(n) = Sum_{i=1..floor((n-1)/2)} i * (n-i) * ((n-i+1) mod 2).

Conjectures from Colin Barker, Nov 20 2017: (Start)

G.f.: 2*x^3*(1 - x + 2*x^2 + 2*x^3 + 2*x^4 + x^6 + x^7) / ((1 - x)^4*(1 + x)^3*(1 + x^2)^3).

a(n) = a(n-1) + 3*a(n-4) - 3*a(n-5) - 3*a(n-8) + 3*a(n-9) + a(n-12) - a(n-13) for n>13.

(End)

EXAMPLE

a(16) = 136; the partitions of 16 into two distinct parts are (15,1), (14,2), (13,3), (12,4), (11,5), (10,6), (9,7). There are 3 partitions with the larger part even, and the sum of the products of the smaller and larger parts is then 14*2 + 12*4 + 10*6 = 136.

MAPLE

A295321:=n->add(i*(n-i)*((n-i+1) mod 2), i=1..floor((n-1)/2)): seq(A295321(n), n=1..100);

MATHEMATICA

Table[Sum[i (n - i) Mod[n - i + 1, 2], {i, Floor[(n - 1)/2]}], {n, 80}]

PROG

(PARI) a(n) = sum(i=1, (n-1)\2, i*(n-i)*((n-i+1) % 2)); \\ Michel Marcus, Mar 26 2018

CROSSREFS

Cf. A295320.

Sequence in context: A120710 A115780 A101189 * A001443 A195287 A070015

Adjacent sequences:  A295318 A295319 A295320 * A295322 A295323 A295324

KEYWORD

nonn,easy

AUTHOR

Wesley Ivan Hurt, Nov 19 2017

STATUS

approved

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Last modified September 23 16:17 EDT 2020. Contains 337314 sequences. (Running on oeis4.)