portrait: 51

a pentagonal number as well as a centered pentagonal number and an 18-gonal number the 6th Motzkin number, telling the number of ways to draw non-intersecting chords between any six points on a circle's boundary, no matter where the points may be located on the boundary. a Perrin number, coming after 22, 29, 39 in the sequence (and the sum of the first two) a Størmer number, since the greatest prime factor of 512 + 1 = 2602 is 1301, which is substantially more than 51 twice.