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A331478 Irregular triangle T(n,k) = n - (s - k + 1)^2 for 1 <= k <= s, with s = floor(sqrt(n)). 1
0, 1, 2, 0, 3, 1, 4, 2, 5, 3, 6, 4, 7, 0, 5, 8, 1, 6, 9, 2, 7, 10, 3, 8, 11, 4, 9, 12, 5, 10, 13, 6, 11, 14, 0, 7, 12, 15, 1, 8, 13, 16, 2, 9, 14, 17, 3, 10, 15, 18, 4, 11, 16, 19, 5, 12, 17, 20, 6, 13, 18, 21, 7, 14, 19, 22, 8, 15, 20, 23, 0, 9, 16, 21, 24, 1 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

Row n begins with n - floor(sqrt(n)).

Zero appears in row n for n that are perfect squares. Let r = sqrt(n). For perfect square n, there exists a partition of n that consists of a run of r parts that are each r themselves; e.g., for n = 4, we have {2, 2}, for n = 9, we have {3, 3, 3}. It is clear through the Ferrers diagram of these partitions that they are equivalent to their Durfee square, thus n - s^2 = 0.

Since the partitions of any n contain Durfee squares in the range of 1 <= s <= floor(sqrt(n)) (with perfect square n also including k = 0), the distinct Durfee square excesses must be the differences n - s^2 for 1 <= s <= floor(sqrt(n)).

We borrow the term "square excess" from A053186(n), which is simply the difference n - floor(sqrt(n)).

Row n of this sequence contains distinct Durfee square excesses among all integer partitions of n (see example below).

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10125 (rows 1 <= n <= 625, flattened)

Eric Weisstein's World of Mathematics, Durfee Square.

FORMULA

Let s = floor(sqrt(n));

T(n,1) = A053186(n) = n - s;

T(n,k) = T(n,1) + partial sums of 2(s - k + 1) + 1 for 2 <= k <= s + 1.

A000196(n) = Length of row n.

A022554(n) = Sum of row n.

Last term in row n = T(n, A000196(n)) = n - 1.

EXAMPLE

Table begins:

   1:  0;

   2:  1;

   3:  2;

   4:  0,  3;

   5:  1,  4;

   6:  2,  5;

   7:  3,  6;

   8:  4,  7;

   9:  0,  5,  8;

  10:  1,  6,  9;

  11:  2,  7, 10;

  12:  3,  8, 11;

  13:  4,  9, 12;

  14:  5, 10, 13;

  15:  6, 11, 14;

  16:  0,  7, 12, 15;

  ...

For n = 4, the partitions are {4}, {3, 1}, {2, 2}, {2, 1, 1}, {1, 1, 1, 1}. The partition {2, 2} has Durfee square s = 2; for all partitions except {2, 2}, we have Durfee square with s = 1. Therefore we have two unique solutions to n - s^2 for n = 4, i.e., {0, 3}, so row 4 contains these values.

MATHEMATICA

Array[# - Reverse@ Range[Sqrt@ #]^2 &, 625] // Flatten

CROSSREFS

Cf. A000196, A022554, A053186, A117522.

Sequence in context: A025636 A025637 A195826 * A097065 A084964 A267182

Adjacent sequences:  A331475 A331476 A331477 * A331479 A331480 A331481

KEYWORD

nonn,easy,tabf

AUTHOR

Michael De Vlieger, Jan 17 2020

STATUS

approved

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Last modified April 14 19:32 EDT 2021. Contains 342951 sequences. (Running on oeis4.)