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A020662 Lexicographically earliest increasing sequence of positive numbers that contains no arithmetic progression of length 9. 28
1, 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 19, 20, 21, 22, 23, 24, 27, 28, 29, 30, 31, 32, 33, 34, 37, 38, 39, 40, 41, 43, 44, 45, 46, 47, 48, 49, 50, 53, 55, 56, 57, 58, 59, 60, 64, 65, 66, 67, 68, 69, 70, 71, 78, 79, 80, 81, 82, 83, 84, 85, 87, 88, 91, 92, 94, 95, 96, 97 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

LINKS

Robert Israel, Table of n, a(n) for n = 1..10000

MAPLE

Noap:= proc(N, m)

# N terms of earliest increasing seq with no m-term arithmetic progression

local A, forbid, n, c, ds, j;

A:= Vector(N):

A[1..m-1]:= <($1..m-1)>:

forbid:= {m}:

for n from m to N do

  c:= min({$A[n-1]+1..max(max(forbid)+1, A[n-1]+1)} minus forbid);

  A[n]:= c;

  ds:= convert(map(t -> c-t, A[m-2..n-1]), set);

  for j from m-2 to 2 by -1 do

    ds:= ds intersect convert(map(t -> (c-t)/j, A[m-j-1..n-j]), set);

    if ds = {} then break fi;

  od;

  forbid:= select(`>`, forbid, c) union map(`+`, ds, c);

od:

convert(A, list)

end proc:

Noap(100, 9); # Robert Israel, Jan 04 2016

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: A023803 A122132 A325389 * A306202 A328335 A302569

Adjacent sequences:  A020659 A020660 A020661 * A020663 A020664 A020665

KEYWORD

nonn

AUTHOR

David W. Wilson

STATUS

approved

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Last modified October 20 10:32 EDT 2019. Contains 328257 sequences. (Running on oeis4.)