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.

Showing 1-1 of 1 results.

A167457 Smallest sequence which lists the position of digits "7" in the sequence.

Original entry on oeis.org

2, 7, 8, 9, 10, 77, 770, 800, 801, 802, 803, 804, 805, 806, 808, 809, 810, 811, 812, 813, 814, 815, 816, 818, 819, 820, 821, 822, 827, 828, 829, 830, 831, 832, 833, 834, 835, 836, 838, 839, 840, 841, 842, 843, 844, 845, 846, 848, 849, 850, 851, 852, 853, 854
Offset: 1

Views

Author

M. F. Hasler, Nov 19 2009

Keywords

Comments

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.

Examples

			We cannot have a(1)=1 (since then there's no "7" in the first place), but a(1)=2 is possible.
Then a(2) must start with a digit "7", so a(2)=7 is the smallest possible choice.
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.
Then the reasoning continues in analogy with A167452-A167456.

Crossrefs

Cf. A098645, A167519, A167520, A167452 - A167456.

Programs

  • 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)) ])
Showing 1-1 of 1 results.

Started in 1964 by Neil J. A. Sloane | Maintained by The OEIS Foundation Inc.

Content is available under The OEIS End-User License Agreement.