This came up as sub-problem during Young-Han’s group meeting today and we mulled over it for a few minutes but didn’t come up with a non-ugly answer. I’m sure, given the number of Real Mathematicians ™ who read this, that someone out there knows of an “obvious” explanation.
Suppose I give you p integers in an arbitrary order (where p is prime). While maintaining the order and using only addition, multiplication, and parenthesis, is it always possible to make an expression which evaluates to 0 mod p?
I think it’s true, but I’m sure there’s some special property of 
An Ergodic Walk 
Not only is it true, but I don’t think p has to be prime.
If you an find a set of consecutive elements that add to 0 mod p, you can add that, put it in parentheses, and multiply it by everything else. We’ll show such a set always exists.
Consider the sums:
a_1
a_1+a_2
a_1+a_2+a_3
…
a_1+a_2+…+a_p
If these p sums are all different mod p, one of them must be 0 mod p. If they’re not all different, take any two that are the same, and their difference is a sum of consecutive integers equal to 0 mod p. Either way, we’re done.
(Now why can’t I answer questions related to my research this quickly?)
Also, p is necessary as well as sufficient (since a sequence of (p-1) 1’s is no good).
Would (1-1).(1+1+…+1) be acceptable when we have(p-1) 1’s?
Subtraction is not allowed…
Yes, I just noticed that subtraction is not allowed: sorry about that. But if p is not a prime, say p = 6, then
(1+1).(1+1+1)
is 0 mod 6, and so (p-1) 1’s might still massaged into 0 mod p.
Well that certainly is simpler than the algorithmic road we were treading down…
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