OFFSET
1,3
COMMENTS
Total surface area of all rectangular prisms with dimensions s X t X t where s and t are positive integers, n = s + t and s < t. For example, the surface area gives 4*s*t + 2*t^2 = 2*t*(2*s+t).
Consider the partitions of n into two distinct parts (s,t) with s < t. Then a(n) is the sum of all the products (2*t)*(2*s+t), using corresponding parts from each (s,t).
Also, the total area of all rectangles with dimensions (2*t) X (2*s+t), where s and t are positive integers, n = s + t and s < t.
LINKS
FORMULA
G.f.: 2*x^3*(8 + 7*x + 6*x^2 + x^3)/((1 + x)^3*(1 - x)^4). - Bruno Berselli, May 10 2019
a(n) = a(n-1) + 3*a(n-2) - 3*a(n-3) - 3*a(n-4) + 3*a(n-5) + a(n-6) - a(n-7).
a(n) = 2 * Sum_{i=1..floor((n-1)/2)} (n - i)*(n + i).
MATHEMATICA
Table[n*(2*n - 3 - (-1)^n)*(11*n + (-1)^n)/24, {n, 60}]
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Wesley Ivan Hurt, May 09 2019
STATUS
approved