Check if number is a sum of consecutive numbers or not

alert(!!((n=~~prompt())&(n-1)))

From my testing, I believe this gives correct solutions.

http://jsfiddle.net/3e9FZ/

Edit: (SPOILER ALERT!) Here is the justification for this answer:

All odd numbers greater than 1 can be trivially written as the sum of two consecutive numbers (ex 15 = 7+8, 23=11+12, etc.).
For even numbers having an odd factor where the odd factor is less than twice the even factor. For example, 4*7, because 7 < (2*4 = 8). Simply add 7 numbers with 4 at the center, 1+2+3+4+5+6+7.
For even numbers having an odd factor where the odd factor is more than twice the even factor. For example, 4*9, because 9 > (2 * 4 = 8). Double the even factor, and halve the odd factor to get 8*4.5. You will add the 8 numbers centered at 4.5, i.e., 1+2+3+4+5+6+7+8.
The only numbers left are the even numbers having no odd factor, i.e., the powers of two. The formula for the sum of a consecutive set of numbers is (avg * count). Now if the count is odd, then avg is a whole number, and (avg * count) has an odd factor. If count is even, then avg must be #.5, and thus avg * 2 is odd, and so avg * count has an odd factor. Therefore, any sum using the formula (avg * count) must have an odd factor, which powers of two do not, and therefore have no solution.

~.(&!!"falsetrue"5/=

Someone had to.

Ruby (23)

only 3 chars longer than golfscript :)

p 0<(n=$*[0].to_i)&n-1

output:

$ ruby gc2958.rb 4
false
$ ruby gc2958.rb 43
true
$ wc gc2958.rb
  1       2      23 gc2958.rb