It is possible that this sequence is two sequences intertwined. Consider every other term: 21, 66, 115, X. The short sequence 21, 66, 115 can be interpreted as a quadratic sequence, which makes the next term 168. Similarly the term after 168 is 252. 252 comes from treating 45, 102, 171 as a quadratic sequence.

*Edited on ***October 21, 2003, 1:53 pm**