Let f (n) be the sum of first n terms of sequence 0, 1, 1, 2, 2, 3, 3, 4, 4, …. Then
(a) f(n) = n2/4 , where n is an even number
(b) f(n) = n2 - 1/4, where n is an odd number
(c) f(n + m) - f(n - m) = nm, where n, m ∈ I+(n > m)
(d) f(n + m) - f(n - m) = 4mn + 1/1, where n,m ∈ I+(n > m)
