Generalizing an old formula

It all started with a discussion about the role of pictures as substitute for calculational proofs. I was reminded of the pictorial illustration of the theorem that the sum of the first  n  odd natural numbers equals n2:

EWD857 figure 1

Immediately the question rose whether this formula can be generalized to higher powers of n, since from the 4th power onwards, visualization breaks down.

By mentally cutting 3 mutually orthogonal slices of thickness  1  from a cube of  n  by  n  by  n , one can convince oneself that

n3 = (S i: 1 ≤ i ≤ n: i2 + i(i-1) + (i-1)2)

and one begins to suspect the validity of

n4 = (S i: 1 ≤ in: i3 + i2(i-1) + i(i-1)2 + (i-1)3)

and similar formula, in general

nk = (S i: 1 ≤ in: (S j: 0 ≤ j < k: ik-1-j(i-1)j)). (0)

This holds provided

nk = (S j: 0 ≤ j < k: nk-1-j(n-1)j) + (n-1)k (1)

because (0) holds for  n=0 , while (1) then provides the step for proving (0) by induction over  n .

We prove (1), which holds for  k=1 , by mathematical induction over  k:
       nk = (S j: 0 ≤ j < k: nk-1-j(n-1)j) + (n-1)k
≡ { multiply both sides by n; n = 1 + (n-1) }
       nk+1 = (S j: 0 ≤ j < k: nk-j(n-1)j) + (n-1)k + (n-1)k+1
≡ { definition of summation and n0 = 1 }
       nk+1 = (S j: 0 ≤ j < k+1: nk+1-1-j(n-1)j) + (n-1)k+1

q.e.d.

The above was just for the record. I find the above double induction satisfactory and formula (0) was new for me.

Observation. In the mean time I asked people from five different countries: none of them had seen the above picture at school. We all agreed that that was a pity. (End of Observation)


Burroughs
Austin Research Center
AUSTIN, Texas
25 June 1983
prof. dr. Edsger W. Dijkstra
Burroughs Research Fellow