Given an array of integers A, give an algorithm to find the longest Arithmetic progression in it

i.e find a sequence i1 < i2 < … < ik, such that
A[i1], A[i2], …, A[ik] forms an arithmetic progression, and k is the largest possible.
The sequence S1, S2, …, Sk is called an arithmetic progression if
Sj+1 – Sj is a constant

http://theory.cs.uiuc.edu/~jeffe/pubs/pdf/arith.pdf