Numbers k >= 1 such that k^2 - r^2 is a repunit (A002275) for some 1 <= r < k.

Terms

a(0) =6a(1) =20a(2) =56a(3) =156a(4) =340a(5) =344a(6) =356a(7) =460a(8) =556a(9) =656a(10) =760a(11) =1444a(12) =1520a(13) =1700a(14) =2444a(15) =2656a(16) =4280a(17) =4440a(18) =5056a(19) =5556a(20) =7320a(21) =7940a(22) =18520a(23) =40620a(24) =55056a(25) =55556a(26) =65656a(27) =76140a(28) =167000a(29) =187856