561

561 = 3 x 11 x 17.

561 is 1000110001 in base 2 (binary) (A006995).

561 is the 33rd triangular number (A000217), the sum of the first 33 integers. It is also a hexagonal number (A000384) and a dodecagonal number (A051624).

561 is the smallest Carmichael number (A002997).

561 is a centered icosahedral number (cuboctahedral number) (A005902).

Source: Number Gossip

793

793 = 13 x 61.

793 is one less than twice its reversal: 793 = 2(397) - 1.

793 is a dodecagonal number and a centered dodecagonal number (A003154).

793 has two representations as a sum of two squares: 793 = 3² + 28² = 8² + 27².

793 is the hypotenuse of two primitive Pythagorean triples: 793² = 168² + 775² = 432² + 665².

Source: What's Special About This Number?

156

156 = 2² x 3 x 13. It is a pronic number because it is the product of two consecutive numbers: 156 = 12 x 13.

156 is 1111 in base 5.

156 is a dodecagonal number.

156 is the number of graphs with six vertices.

A clock will strike 156 times in the course of a day.

Source: Number Gossip