Triangular numbers with digits in nondecreasing order.

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =15a(5) =28a(6) =36a(7) =45a(8) =55a(9) =66a(10) =78a(11) =136a(12) =378a(13) =666a(14) =1128a(15) =1225a(16) =1378a(17) =2278a(18) =2346a(19) =2556a(20) =5778a(21) =12246a(22) =13366a(23) =22366a(24) =22578a(25) =35778a(26) =111156a(27) =222778a(28) =223446a(29) =333336