Index of smallest pentagonal number with n digits.

Terms

a(0) =1a(1) =3a(2) =9a(3) =26a(4) =82a(5) =259a(6) =817a(7) =2583a(8) =8166a(9) =25821a(10) =81650a(11) =258200a(12) =816497a(13) =2581990a(14) =8164966a(15) =25819890a(16) =81649659a(17) =258198890a(18) =816496582a(19) =2581988898a(20) =8164965810a(21) =25819888975a(22) =81649658093a(23) =258198889748a(24) =816496580928