A378379 - OEIS

A378379

Minimal x such that there is a partition of (x, x) into sums of distinct pairs of nonnegative integers with size at least n, excluding (0, 0).

1, 1, 2, 3, 4, 6, 7, 9, 10, 12, 14, 16, 18, 20, 23, 25, 28, 30, 33, 35, 38, 41, 44, 47, 50, 53, 56, 60, 63, 67, 70, 74, 77, 81, 84, 88, 92, 96, 100, 104, 108, 112, 116, 120, 125, 129, 134, 138, 143, 147, 152, 156, 161, 165, 170, 175, 180, 185, 190, 195, 200, 205, 210, 215, 220

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OFFSET

1,3

COMMENTS

For (n, n), there is at least one maximal partition P that's symmetric: (x, y) in P <=> (y, x) in P. This can be proven by manipulating integer sequences c(i) (i >= 1) such that 0 <= c(i) <= i+1 for all i and Sum_{i > 0} i*c(i) = 2n, which correspond to partitions P of (n, n) with size |P| = Sum_{i > 0} c(i), where c(i) is equal to number of (x, y) in P such that x+y = i.

LINKS

Table of n, a(n) for n=1..65.

FORMULA

a(n*(n+3)/2) = n*(n+1)*(n+2)/6.

EXAMPLE

For n = 8, a(n) = 9, as (9, 9) can be expressed as the sum (0, 1) + (0, 2) + (0, 3) + (1, 0) + (2, 0) + (3, 0) + (1, 2) + (2, 1), but the longest sum for (8, 8) has 7 pairs.

PROG

(Python)

import math

def A378379(n: int) -> int:

l = (math.isqrt(1+8*n)-1)//2 # l = A003056(n), min. possible largest pair norm

r = n - (l-1)*(l+2)//2 # r = n - A000096(l-1), number of pairs with norm l

return ((l-1)*l*(l+1)//3 + l*r + 1)//2 # ceil((A007290(l+1) + l*r) / 2)

CROSSREFS

Maximal size among partitions considered by A054242 and A201377.

Minimal x such that A378126(x, x) >= n.

Cf. A086435.

Sequence in context: A248635 A171511 A225819 * A205805 A246372 A006254

Adjacent sequences: A378376 A378377 A378378 * A378380 A378381 A378382

KEYWORD

nonn

AUTHOR

Jimin Park, Nov 24 2024

STATUS

approved