cp's OEIS Frontend

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.

A167455 Smallest sequence which lists the position of digits "5" in the sequence.

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%I A167455 #4 Apr 04 2015 21:34:04
%S A167455 2,5,6,7,55,56,60,61,62,63,64,66,67,68,69,70,71,72,73,74,76,77,78,79,
%T A167455 80,81,82,83,84,550,605,5555,6555,55555,56555,555555,600000,600001,
%U A167455 600002,600003,600004,600006,600007,600008,600009,600010,600011,600012,600013
%N A167455 Smallest sequence which lists the position of digits "5" in the sequence.
%C A167455 The lexicographically earliest sequence such that a(1),a(2),a(3),... is the (increasing) list of the positions of digits "5" in the string obtained by concatenating all these terms, written in base 10.
%e A167455 We cannot have a(1)=1 (since then there's no "5" in the first place), but a(1)=2 is possible.
%e A167455 Then a(2) must start with a digit "5", so a(2)=5 is the smallest possible choice.
%e A167455 This allows us to go on with a(3)=6, a(4)=6, but then must be follow 3 digits "5" (the 5th, 6th and 7th digit of the sequence), so a(5)=55 and a(6)=56 are the smallest possible choice.
%e A167455 The reasoning continues in analogy with A167452-A167454.
%o A167455 (PARI) concat([ [2,5,6,7,55,56], vector((55-8)\2,i,60-(i<=5)+i+(i>=15)), [550, 605, 5555, 6555, 55 555, 56 555, 555 555], select(x->x%10-5 & x\10%10-5,vector((550-84)\6+10,i,600 000+i-1)) ])
%Y A167455 Cf. A098645, A167519, A167520, A167452, A167453, A167454.
%K A167455 base,nonn
%O A167455 1,1
%A A167455 _M. F. Hasler_, Nov 19 2009