MathDB
Putnam 2009 B6

Source:

December 7, 2009
Putnaminductioncollege contests

Problem Statement

Prove that for every positive integer n, n, there is a sequence of integers a0,a1,,a2009 a_0,a_1,\dots,a_{2009} with a_0\equal{}0 and a_{2009}\equal{}n such that each term after a0 a_0 is either an earlier term plus 2k 2^k for some nonnnegative integer k, k, or of the form bmodc b\mod{c} for some earlier positive terms b b and c. c. [Here bmodc b\mod{c} denotes the remainder when b b is divided by c, c, so 0(bmodc)<c. 0\le(b\mod{c})<c.]