217

217 is a centered hexagonal number, a 12-gonal number, a centered 36-gonal number, a Fermat pseudoprime to base 5, and a Blum integer.It is both the sum of two positive cubes and the difference of two positive consecutive cubes in exactly one way: 217 = 6 3 + 1 3 = 9 3 − 8 3 {\displaystyle 217=6^{3}+1^{3}=9^{3}-8^{3}}. When written in binary, it is a non-repetitive Kaprekar number.[2] It is also the sum of all the divisors of 100.