Palindromic Triangular Numbers

Sequence of Palindromic Triangular Numbers
The sequence of triangular numbers which are also palindromic begins:
 * $1, 3, 6, 55, 66, 171, 595, 666, 3003, \ldots$

Their indices are:
 * $1, 2, 3, 10, 11, 18, 34, 36, 77, \ldots$