This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A139121 #12 Mar 12 2015 18:54:09
%S A139121 0,4,10,13,19,26,34,46,57,70,81,94,113,123,137,151,168,184,205,217,
%T A139121 232,250,267,287,310,322,340,357,379,403,418,435,455,478,503,516,529,
%U A139121 546,565,585,608,619,633,651,671,692,715,729,746,765,785,808,820,833,852,873,895,920,933,952,973,995,1020
%N A139121 Total number of letters in the preceding terms spelled out in French.
%C A139121 Form a sequence of French words as follows: look to the left, towards the beginning of the sequence and write down the number of letters you see; repeat; then replace the words by the corresponding numbers.
%C A139121 The sequence of words is: zero, quatre, dix, treize, dix-neuf, vingt-six, trente-quatre, quarante-six, cinquante-sept, ...
%C A139121 Hyphens, accents and spaces are not counted.
%C A139121 For an English version see A139097.
%D A139121 E. Angelini, "Jeux de suites", in Dossier Pour La Science, pp. 32-35, Volume 59 (Jeux math'), April/June 2008, Paris.
%e A139121 The first word is "zero", because initially there are no letters to the left. The second word is "quatre" (and so a(2)=4), because at the end of the first word we can see four letters to the left. And so on.
%o A139121 (PARI) A139121(n)={ n>1 & return( #select(Vec(French(n=A139121(n-1))),x->x>"@")+n )} /* see A167507 for French() */ \\ _M. F. Hasler_, Sep 29 2011
%Y A139121 Cf. A005589, A060403, A139097.
%K A139121 nonn,word
%O A139121 1,2
%A A139121 _N. J. A. Sloane_ (based on Angelini's article), Jun 08 2008, Jun 15 2008
%E A139121 Offset and a(9) corrected (according to wording of example) and terms beyond a(9) from _M. F. Hasler_, Sep 29 2011