# Centered triangular number

A centered (or centred) triangular number is a centered figurate number that represents a triangle with a dot in the center and all other dots surrounding the center in successive triangular layers. The centered triangular number for n is given by the formula

${{3n^2 + 3n + 2} \over 2}.$

The following image shows the building of the centered triangular numbers using the associated figures: at each step the previous figure, shown in red, is surrounded by a triangle of new points, in blue.

The first few centered triangular numbers are:

1, 4, 10, 19, 31, 46, 64, 85, 109, 136, 166, 199, 235, 274, 316, 361, 409, 460, 514, 571, 631, 694, 760, 829, 901, 976, 1054, 1135, 1219, 1306, 1396, 1489, 1585, 1684, 1786, 1891, 1999, 2110, 2224, 2341, 2461, 2584, 2710, 2839, 2971, … (sequence A005448 in OEIS).

Each centered triangular number from 10 onwards is the sum of three consecutive regular triangular numbers. Also each centered triangular number has a remainder of 1 when divided by three and the quotient (if positive) is the previous regular triangular number.

The sum of the first n centered triangular numbers is the magic constant for an n by n normal magic square for n > 2.

## Centered triangular prime

A centered triangular prime is a centered triangular number that is prime. The first few centered triangular primes are

19, 31, 109, 199, 409, … (sequence A125602 in OEIS).

(corresponding to n: 3, 4, 8, 11, 16, ...)