Numbers k such that k*(k - 1)/2 is a square.

Terms

a(0) =1a(1) =2a(2) =9a(3) =50a(4) =289a(5) =1682a(6) =9801a(7) =57122a(8) =332929a(9) =1940450a(10) =11309769a(11) =65918162a(12) =384199201a(13) =2239277042a(14) =13051463049a(15) =76069501250a(16) =443365544449