Numbers k such that k and the k-th triangular number T(k) = k*(k+1)/2 have only odd digits.

Terms

a(0) =1a(1) =5a(2) =13a(3) =17a(4) =177a(5) =1777a(6) =3937a(7) =5537a(8) =5573a(9) =15173a(10) =55377a(11) =55733a(12) =79137a(13) =135173a(14) =195937a(15) =339173a(16) =377777a(17) =399377a(18) =791377a(19) =3397973a(20) =5199137a(21) =7913777a(22) =13535137a(23) =17397537a(24) =33993973a(25) =37735377a(26) =39993777a(27) =59591173a(28) =59919137a(29) =79971937