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