login
Each of the numbers describes the previous number, and is described by the next number.
0

%I #14 May 06 2022 11:10:26

%S 1,11,12,1121,1321,123121,132231,123222,112431,13214131,14322141,

%T 13423122,12324123,12233241,12233241,12233241,12233241,12233241,

%U 12233241,12233241,12233241,12233241,12233241,12233241,12233241,12233241,12233241,12233241,12233241,12233241

%N Each of the numbers describes the previous number, and is described by the next number.

%C Starting with 1, each of the rest describes the previous number in a "digit + number_of_digits" manner. The odd-indexed digits are digits that appeared in the previous term, ordered by the first appearance. The even-indexed digits are how many times that digit appeared. [Edited by _Michel Marcus_, Oct 26 2021]

%t NestList[FromDigits@Flatten@Tally@IntegerDigits@#&,1,30] (* _Giorgos Kalogeropoulos_, Oct 26 2021 *)

%o (PARI) lista(nn) = {my(v = vector(nn)); v[1] = 1; for (n=2, nn, my(vd = digits(v[n-1]), vu = Set(vd), vw = vector(#vu), pos=0, ss=""); for (i=1, #vd, if (!setsearch(Set(vw), vd[i]), pos++; vw[pos] = vd[i]);); for (i=1, #vw, ss = concat(ss, vw[i]); ss = concat(ss, #select(x->(x==vw[i]), vd));); v[n] = eval(ss);); v;} \\ _Michel Marcus_, Oct 26 2021

%Y A variant of A005150. See Index entries under "Say what you see". - _N. J. A. Sloane_, May 10 2009

%Y Cf. A121808.

%K nonn,base

%O 1,2

%A Adrian Lee (prof.adrian.lee(AT)gmail.com), May 07 2009

%E More terms from _Michel Marcus_, Oct 26 2021