55five55 is a square pyramidal number. 55 is: . the 10th Fibonacci number [1] and the 10th triangular number, [2] The sum of 55's dits is also 10.; the 5th heptagonal number, [3] the 5th squareDiscover luxury apartments in the heart of FiDi Manhattan. 55 Broad in FiDi is here to offer you a superior living experience.