651

651 = 3 x 7 x 31.

651 is a pentagonal number and a nonagonal number. It is the smallest number that is both pentagonal and nonagonal.

651 is 10101 in base 5 and 3003 in base 6.

651 is the number of binary trees of height 4.

651 has a fourth power that is the sum of four fourth powers.

101,556 = 372 x 273 = 651 x 156.

Source: Number Gossip