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A176541 Numbers n such that there exist n consecutive triangular numbers which sum to a square. 9
0, 1, 2, 3, 4, 11, 13, 22, 23, 25, 27, 32, 37, 39, 46, 47, 48, 49, 50, 52, 59, 66, 71, 73, 83, 94, 98, 100, 104, 107, 109, 111, 118, 121, 128, 143, 146, 147, 148, 157, 167, 176, 179, 181, 183, 191, 192, 193, 194, 200, 214, 219, 227, 239, 241, 242, 243, 244, 253, 263 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Numbers n such that there exists some x>=0, such that A000292(x+n) - A000292(x) is a square. Terms of this sequence, for which only a finite number of solutions x exists, are given in A176542.

Integer n is in the sequence if there exists non-degenerate solutions to the diophantine equation: 8x^2 - n*y^2 - A077415(n) = 0. A degenerate solution is one involving triangular numbers with negative indexes.

The sum of n consecutive triangular numbers starting at the j'th is sum_{k=j..j+n-1} A000217(k) = n*(n^2+3*j*n+3*j^2-1)/6, see A143037. - R. J. Mathar, May 06 2015

LINKS

Table of n, a(n) for n=1..60.

EXAMPLE

0 is in the sequence because the sum of 0 consecutive triangular numbers is 0 (a square)

1 is in the sequence because there exist triangular numbers which are squares (cf. A001110)

2 is in the sequence because ANY 2 consecutive triangular numbers sum to a square.

3 is in the sequence because there are infinitely many solutions (cf. A165517).

4 is in the sequence because there infinitely many solutions (cf. A202391).

5 is NOT in the sequence because no 5 consecutive triangular numbers sum to a square.

For n=8, solutions to the Diophantine equation exist, but start at A000217(-2) and A000217(-6): 1 + 0 + 0 + 1 + 3 + 6 + 10 + 15 = 36 and 15 + 10 + 6 + 3 + 1 + 0 + 0 + 1 = 36. There are no non-degenerate solutions for n=8. Hence, 8 is not included in the sequence.

For n=11, there exist infinitely many solutions (cf. A116476).

CROSSREFS

Cf. A176542, A000217, A000292, A001110, A077415.

Sequence in context: A155768 A138985 A184806 * A171376 A141704 A061919

Adjacent sequences:  A176538 A176539 A176540 * A176542 A176543 A176544

KEYWORD

nonn

AUTHOR

Andrew Weimholt, Apr 20 2010

EXTENSIONS

More terms from Max Alekseyev, May 10 2010

STATUS

approved

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Last modified November 22 05:36 EST 2017. Contains 295076 sequences.