Position of n-th 6 in A031087.

Terms

a(0) =6a(1) =21a(2) =39a(3) =57a(4) =75a(5) =93a(6) =100a(7) =102a(8) =104a(9) =106a(10) =108a(11) =110a(12) =111a(13) =112a(14) =114a(15) =116a(16) =129a(17) =147a(18) =171a(19) =198a(20) =225a(21) =252a(22) =279a(23) =306a(24) =316a(25) =319a(26) =322a(27) =325a(28) =328a(29) =331