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 A167457 #4 Apr 04 2015 21:34:04 %S A167457 2,7,8,9,10,77,770,800,801,802,803,804,805,806,808,809,810,811,812, %T A167457 813,814,815,816,818,819,820,821,822,827,828,829,830,831,832,833,834, %U A167457 835,836,838,839,840,841,842,843,844,845,846,848,849,850,851,852,853,854 %N A167457 Smallest sequence which lists the position of digits "7" in the sequence. %C A167457 The lexicographically earliest sequence such that a(1),a(2),a(3),... is the (increasing) list of the positions of digits "7" in the string obtained by concatenating all these terms, written in base 10. %e A167457 We cannot have a(1)=1 (since then there's no "7" in the first place), but a(1)=2 is possible. %e A167457 Then a(2) must start with a digit "7", so a(2)=7 is the smallest possible choice. %e A167457 This allows us to go on with a(3)=8, a(4)=9, a(5)=10, but then must be follow 4 digits "6" (the 7th through 10th digit of the sequence), so a(6)=77 and a(7)=770 are the smallest possible choices. %e A167457 Then the reasoning continues in analogy with A167452-A167456. %o A167457 (PARI) concat([ [2,7,8,9,10,77,770], vector((77-10)\3-1,i,800-(i<=7)+i+(i>=17)), [827], select(x->x%10-7 & x\10%10-7,vector((770-77)\3+20,i,827+i)) ]) %Y A167457 Cf. A098645, A167519, A167520, A167452 - A167456. %K A167457 base,nonn %O A167457 1,1 %A A167457 _M. F. Hasler_, Nov 19 2009