A PRIMER OF ANALYTIC NUMBER THEORY: From Pythagoras to ...

12 1. Sums and Differences
square numbers as functions and not sequences. So,
s(n) = n 2 ,
�s(n) = (n + 1) 2 − n 2
= n 2 + 2n + 1 − n 2 = 2n + 1,
� 2 s(n) = (2(n + 1) + 1) − (2n + 1) = 2.
Based on the pattern of second differences, we expect that the pentagonal
numbers, p(n), should satisfy � 2 p(n) = 3 for all n. This means that �p(n) =
3n + C for some constant C, since
�(3n + C) = (3(n + 1) + C) − (3n + C) = 3.
What about p(n) itself? To correspond to the +C term, we need a term,
Cn + D for some other constant D, since
�(Cn + D) = (C(n + 1) + D) − (Cn + D) = C.
We also need a term whose difference is 3n. We already observed that for the
triangular numbers, �t(n) = n + 1. So, �t(n − 1) = n and �(3t(n − 1)) =
3n. So,
p(n) = 3t(n − 1) + Cn + D = 3(n − 1)n/2 + Cn + D
for some constants C and D. We expect p(1) = 1 and p(2) = 5, because they
are pentagonal numbers; so, plugging in, we get
0 + C + D = 1,
3 + 2C + D = 5.
Solving, we get that C = 1 and D = 0, so
p(n) = 3(n − 1)n/2 + n = n(3n − 1)/2.
This seems to be correct, since it gives
p(n) : 1 5 12 22 35 ...,
�p(n) : 4 7 10 13 16 ...,
� 2 p(n) : 3 3 3 3 3 ....
Exercise 1.2.1. Imitate this argument to get a formula for the hexagonal
numbers, h(n).