A337945 Numbers m with a solution (s,t,k) such that s^2 + t^2 = k*m, s + t = m, 1 <= s <= t and 1 <= k <= m - 1.

2, 4, 6, 8, 9, 10, 12, 14, 16, 18, 20, 22, 24, 25, 26, 27, 28, 30, 32, 34, 36, 38, 40, 42, 44, 45, 46, 48, 49, 50, 52, 54, 56, 58, 60, 62, 63, 64, 66, 68, 70, 72, 74, 75, 76, 78, 80, 81, 82, 84, 86, 88, 90, 92, 94, 96, 98, 99, 100, 102, 104, 106, 108, 110, 112, 114, 116, 117, 118, 120

Offset

1,1

Links

Table of n, a(n) for n=1..70.

Formula

k is a term <=> Sum_{i=1..floor(k/2)} Sum_{j=1..k-1} KroneckerDelta((i^2 + (k - i)^2)/j, k) > 0.

Conjecture: k is a term <=> k * Clausen(k, 1) <> 2 * Clausen(k, 0), (Clausen = A160014). In other words: k is in this sequence iff it is not an odd squarefree number. - Peter Luschny, Jun 08 2023

Example

8 is in the sequence since it has the solutions (s,t,k) = (4,4,4) and (2,6,5) such that s^2 + t^2 = k*m, s + t = m, 1 <= s <= t and 1 <= k <= m - 1.
9 is in the sequence since it has the solution (s,t,k) = (3,6,5) such that s^2 + t^2 = k*m, s + t = m, 1 <= s <= t and 1 <= k <= m - 1.

Maple

# Quite inefficient compared to the conjectured formula.
KD := (n, k) -> Physics:-KroneckerDelta[n, k]:
S := k -> local i, j; add(add(KD((i^2 + (k - i)^2)/j , k), j = 1..k-1),
i = 1..floor(k/2)): select(k -> S(k) > 0, [seq(k, k = 1..40)]); # Peter Luschny, Jun 08 2023

Mathematica

Table[If[Sum[Sum[KroneckerDelta[(i^2 + (n - i)^2)/k, n], {k, n - 1}], {i, Floor[n/2]}] > 0, n, {}], {n, 120}] // Flatten

Crossrefs

Cf. A337101, A160014, A039956.

Sequence in context: A353683 A296214 A325572 * A372771 A247423 A182339

Adjacent sequences: A337942 A337943 A337944 * A337946 A337947 A337948

Keyword

nonn

Author

Wesley Ivan Hurt, Oct 01 2020

Status

approved