A triangular number is the number of dots in an equilateral triangle evenly filled with dots. For example, three dots can be arranged in a triangle; thus three is a triangle number. The -th triangle number is the number of dots in a triangle with dots on a side. A triangle number is, equivalently, the sum of the natural numbers from 1 to . The sequence of triangular numbers (sequence A000217 in OEIS) for is: 1, 3, 6, 10, 15, 21, 28, 36, 45, 55, ....