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A308639
a(n) is the number of pairs (i,j) such that 0 < i < j < n-1 and the points (i, a(i)), (j, a(j)) and (n-1, a(n-1)) are aligned.
2
0, 0, 0, 1, 0, 3, 1, 0, 6, 3, 1, 1, 3, 1, 6, 1, 10, 0, 11, 2, 2, 2, 2, 5, 0, 15, 1, 16, 0, 21, 1, 22, 2, 7, 1, 29, 2, 11, 0, 31, 2, 16, 1, 36, 9, 0, 38, 3, 5, 2, 21, 0, 45, 4, 3, 11, 3, 12, 1, 45, 0, 56, 0, 69, 1, 56, 3, 16, 4, 5, 3, 25, 1, 69, 1, 79, 0, 82, 1
OFFSET
1,6
COMMENTS
This sequence is unbounded: by contradiction:
- if the sequence was bounded, say a(n) <= M for any n > 0, then some value, say v, would appear infinitely many times, say at indices (b(1), b(2), ...),
- hence for any k > 0, a(b(k)+1) >= (k-1)*(k-2)/2,
- and for k > 2 + sqrt(2*M), a(b(n)+1) > M , a contradiction, QED.
LINKS
EXAMPLE
The first terms, alongside the pairs (i,j) such that 0 < i < j < n-1 and the points (i, a(i)), (j, a(j)) and (n-1, a(n-1)) are aligned, are:
n a(n) (i,j)'s
-- ---- -----------------------------------
1 0 none
2 0 none
3 0 none
4 1 (1,2)
5 0 none
6 3 (1,2), (1,3), (2,3)
7 1 (3,4)
8 0 none
9 6 (1,2), (1,3), (1,5), (2,3), (2,5), (3,5)
10 3 (3,4), (3,6), (4,6)
11 1 (1,4)
12 1 (4,7)
13 3 (4,7), (4,11), (7,11)
14 1 (6,10)
PROG
(C) See Links section.
CROSSREFS
See A308638 for a similar sequence.
Sequence in context: A371995 A071417 A096653 * A348191 A213668 A256595
KEYWORD
nonn
AUTHOR
Rémy Sigrist, Jun 13 2019
STATUS
approved