What's unique about 1, 81, 1458 and 1729?

They are the only four numbers whose digits when added and multiplied by the reverse of their sum yields the original number.

1:         1 * 1 = 1

81:       8 + 1 = 9 * 9 = 81

1458:   1 + 4 + 5 + 8 = 18 * 81 = 1458

1729:   1 + 7 + 2 + 9 = 19 * 91 = 1729