Y by Adventure10, Mango247
This problem came up during a discussion session at a conference I attended. It looked easy and olympiad-like. But I cannot come up with a solution. I feel like I am missing something trivial.
Let
be a positive integer and
be a subset of
such that the sum of the elements of
is not divisible by
. Prove that the elements of
can be ordered in a way such that no consecutive block of the ordered set
has sum divisible by
.
For example, if
and
, then we can order
as
, and no consecutive block of integers has sum divisible by
.
Let








For example, if




