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A143714 Number of pairs (a,b), 1 <= a <= b <= n, such that (a+b)^2+n^2 is a square. 3
0, 0, 2, 1, 0, 3, 0, 4, 4, 0, 0, 11, 0, 0, 10, 8, 0, 7, 0, 17, 18, 0, 0, 28, 0, 0, 10, 16, 0, 19, 0, 15, 18, 0, 6, 33, 0, 0, 14, 42, 0, 35, 0, 16, 42, 0, 0, 77, 0, 0, 18, 19, 0, 19, 24, 53, 20, 0, 0, 120, 0, 0, 60, 29, 30, 34, 0, 25, 24, 12, 0, 114, 0, 0, 46, 28, 18, 27, 0, 103, 28, 0, 0, 140 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Also: Number of cuboids of maximal side length n having integral shortest path going on the surface from one vertex to the opposite one.

Also: Number of subsets {a,b} of {1,..,n} such that (a+b,n) form the shorter two legs of a Pythagorean triple.

For all primes p, p > 3: a(p)=0 (this directly follows from SierpiƄski's proof that one of the shorter sides of a pythogarean triple must be a multiple of 3, and one must be a multiple of 4). - Michael Turniansky, Jul 27 2016

LINKS

Table of n, a(n) for n=1..84.

Project Euler, Problem 86

EXAMPLE

For n=3, we have the 3 X 3 X 1 and the 3 X 2 X 2 cuboid, for which the shortest path on the surface from one vertex to the opposite is of integral length sqrt(3^2 + (2+2)^2) = sqrt(3^2 + (3+1)^2) = 5.

For n=4, there is the 4 X 2 X 1 cuboid having this property.

For n=1,2 and 5 there is no cuboid having this property, i.e., no s >= 2, s <= 2n such that s^2 + n^2 would be a square.

PROG

(PARI) A143714(M)=sum(a=1, M, sum(b=a, M, issquare((a+b)^2+M^2)))

CROSSREFS

Cf. A143715 (partial sums).

Sequence in context: A098493 A058560 A131047 * A004172 A238942 A082754

Adjacent sequences:  A143711 A143712 A143713 * A143715 A143716 A143717

KEYWORD

easy,nonn

AUTHOR

M. F. Hasler, Aug 29 2008

STATUS

approved

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Last modified January 16 15:53 EST 2019. Contains 319195 sequences. (Running on oeis4.)