a(1) = 1; for n > 1, a(n) = smallest triangular number which is n times another triangular number > 1, or -1 if no such number exists.

A077672

a(1) = 1; for n > 1, a(n) = smallest triangular number which is n times another triangular number > 1, or -1 if no such number exists.

Terms

    a(0) =1a(1) =6a(2) =45a(3) =-1a(4) =15a(5) =36a(6) =21a(7) =120a(8) =-1a(9) =210a(10) =66a(11) =36a(12) =78a(13) =210a(14) =45a(15) =-1a(16) =1326a(17) =378a(18) =190a(19) =120a(20) =210a(21) =66a(22) =1035a(23) =4560a(24) =-1a(25) =78a(26) =1485a(27) =1540a(28) =435a(29) =300

External references