Numbers k such that k = Sum_{i=x..y} i and, in decimal, k is the concatenation of x and y.

Terms

a(0) =15a(1) =27a(2) =429a(3) =1353a(4) =1863a(5) =3388a(6) =3591a(7) =7119a(8) =78403a(9) =133533a(10) =178623a(11) =2282148a(12) =2732353a(13) =3882813a(14) =7103835a(15) =13335333a(16) =17016076a(17) =17786223a(18) =27377889a(19) =32738728a(20) =35639163a(21) =308725039a(22) =347826603a(23) =1248851513a(24) =1333353333a(25) =1420855168a(26) =1777862223a(27) =3146385338a(28) =3699393633