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A066369 Number of subsets of {1, ..., n} with no four terms in arithmetic progression. 1
1, 2, 4, 8, 15, 29, 56, 103, 192, 364, 668, 1222, 2233, 3987, 7138, 12903, 22601, 40200, 71583, 125184, 218693, 386543, 670989, 1164385, 2021678, 3462265, 5930954, 10189081, 17266616, 29654738, 50912618 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

LINKS

Table of n, a(n) for n=0..30.

EXAMPLE

a(5) = 29 because there are 32 subsets and three of them contain four terms in arithmetic progression: {1, 2, 3, 4}, {2, 3, 4, 5} and {1, 2, 3, 4, 5}.

PROG

(Python)

def noap4(n):

.avoid=list()

.for skip in range(1, (n+2)//3):

..for start in range (1, n+1-3*skip):

...avoid.append(set({start, start+skip, start+2*skip, start+3*skip}))

.s=list()

.for i in range(4):

..for smallset in comb(range(1, n+1), i):

...s.append(smallset)

.for i in range(4, n+1):

..for temptuple in comb(range(1, n+1), i):

...tempset=set(temptuple)

...status=True

...for avoidset in avoid:

....if avoidset <= tempset:

.....status=False

.....break

...if status:

....s.append(tempset)

.return s

#Counts all such sets

def a(n):

.return len(noap4(n)) #-David Nacin, Mar 05 2012

CROSSREFS

Cf. A051013,A018789

Sequence in context: A208976 A224959 A108564 * A239555 A000078 A176503

Adjacent sequences:  A066366 A066367 A066368 * A066370 A066371 A066372

KEYWORD

nonn

AUTHOR

Jan Kristian Haugland (jankrihau(AT)hotmail.com), Dec 22 2001

STATUS

approved

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Last modified December 18 10:16 EST 2014. Contains 252138 sequences.