Definition:Ramanujan-Nagell Number

Theorem
A Ramanujan-Nagell number is a positive integer of the form $2^m - 1$ which is also triangular.

Also known as
A Ramanujan-Nagell number is also known as a triangular Mersenne number.

However, this uses the definition of a Mersenne number which is of the form $2^m - 1$ for all positive integers $m$, which differs from the preferred definition on which limits $m$ to the set of prime numbers.

Also see

 * Five Ramanujan-Nagell Numbers