A020657 - OEIS

A020657

Lexicographically earliest increasing sequence of nonnegative numbers that contains no arithmetic progression of length 7.

0, 1, 2, 3, 4, 5, 7, 8, 9, 10, 11, 12, 14, 15, 16, 17, 18, 19, 21, 22, 23, 24, 25, 26, 28, 29, 30, 31, 32, 33, 35, 36, 37, 38, 39, 40, 49, 50, 51, 52, 53, 54, 56, 57, 58, 59, 60, 61, 63, 64, 65, 66, 67, 68, 70, 71, 72, 73, 74, 75, 77, 78, 79, 80, 81, 82, 84, 85

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OFFSET

1,3

COMMENTS

Also the set of numbers with no "6" in their base-7 representation; see Gerver-Ramsey, also comments in A020654. - Nathaniel Johnston, Jun 27 2011

Up to the offset, identical to A037470. There are lexicographically earlier, but non-monotonic sequences which do not contain a 7-term AP, e.g., starting with 0,0,0,0,0,0,1,0,... - M. F. Hasler, Oct 05 2014

LINKS

Nathaniel Johnston, Table of n, a(n) for n = 1..10000

J. L. Gerver and L. T. Ramsey, Sets of integers with no long arithmetic progressions generated by the greedy algorithm, Math. Comp., 33 (1979), 1353-1359.

MAPLE

seq(`if`(numboccur(6, convert(n, base, 7))=0, n, NULL), n=0..85); # Nathaniel Johnston, Jun 27 2011

MATHEMATICA

Select[Range[0, 100], FreeQ[IntegerDigits[#, 7], 6]&] (* Jean-François Alcover, Jan 27 2023 *)

PROG

(PARI) a(n)=vector(#n=digits(n-1, 6), i, 7^(#n-i))*n~ \\ M. F. Hasler, Oct 05 2014

CROSSREFS

Summary of increasing sequences avoiding arithmetic progressions of specified lengths (the second of each pair is obtained by adding 1 to the first):

3-term AP: A005836 (>=0), A003278 (>0);

4-term AP: A005839 (>=0), A005837 (>0);

5-term AP: A020654 (>=0), A020655 (>0);

6-term AP: A020656 (>=0), A005838 (>0);

7-term AP: A020657 (>=0), A020658 (>0);

8-term AP: A020659 (>=0), A020660 (>0);

9-term AP: A020661 (>=0), A020662 (>0);

10-term AP: A020663 (>=0), A020664 (>0).

Sequence in context: A346151 A247832 A047368 * A037470 A187394 A057907

Adjacent sequences: A020654 A020655 A020656 * A020658 A020659 A020660

KEYWORD

nonn,easy

AUTHOR

David W. Wilson

EXTENSIONS

Name edited by M. F. Hasler, Oct 10 2014. Further edited by N. J. A. Sloane, Jan 04 2016

STATUS

approved