P13 · Packing and covering · Classic · Hard

Packing circles of radius 1,2,…,n into a square · n = 13

Also known aspacking circles with radii 1 to n in a squaregraduated circle packing

Pack n non-overlapping circles of radii 1, 2, …, n inside an enclosing square to minimize its side length.

Instancen = 13

ObjectiveMinimize square side

Record comparison

square side · Lower is better

Record	Value / interval	Author / holder	Source
External best known	[55.99136153895, 55.99136153905]	Eckard Specht2005–2026	Packomania ↗
Site record	No player record yet	—	—

Formal definition

ContainerA square [0, side] × [0, side] with the origin at its lower-left corner, where the smallest circle has unit radius 1 and side is the objective value to minimize
SubmissionSquare side and centers, listed in order of radii 1, 2, …, n
ConstraintsCircle i has radius i; no two interiors overlap; every circle lies wholly inside the square container
ObjectiveMinimize the side length of the enclosing square

Open full editor ↗

Layout shown: starter layout

55.99136154y0

12345678910111213

0x55.99136154

VERIFIED CONSTRUCTIONside = 55.99136154

Getting a feel for it

Corner placement and interstitial gaps

Because a square has four right-angle corners, seating the largest few circles into the corners and staggering them across the diagonals is key to minimizing the side length, while smaller circles fit into the triangular pockets left behind. Increasing n by 1 introduces a new largest circle that often forces a reassignment of the four corners.

Proven optima and open frontier

Optimality for n ≤ 4 follows from elementary center-distance projections among the largest circles (see the instance notes). Packomania's csqn table lists best known values up to n = 100, and its ratio column is this problem's side. The table marks none of them proved, so n ≥ 5 remains open. The circular-container counterpart was a Zimmermann contest problem, also on Packomania.

Source

LAYOUT & HISTORY

Starter layout

Inspect the layout, solver notes and record history.

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ANSWER FORMAT

How to write your answer

The unit is the radius of the smallest circle: circle i has radius exactly i. The container is the square you name — lower-left corner at the origin (0, 0), upper-right at (side, side) — and a smaller side scores better. Coordinates and radii share one unit and are written as plain decimals such as "3.5", to at most nine decimal places.

Submit the square side length and the circle centers listed in order of radii 1, 2, …, n. Write each number as a decimal string such as "3.5".

Starter answer

{
  "centers": [
    [
      "42.618729721",
      "12.546346412"
    ],
    [
      "23.432749791",
      "53.968925233"
    ],
    [
      "14.932881594",
      "26.792121959"
    ],
    [
      "4.146767156",
      "26.75153379"
    ],
    [
      "32.282868199",
      "38.694808225"
    ],
    [
      "30.544956025",
      "49.99136154"
    ],
    [
      "48.939187002",
      "7.052174538"
    ],
    [
      "26.042855313",
      "26.240723811"
    ],
    [
      "32.784609691",
      "9"
    ],
    [
      "45.99136154",
      "45.819005289"
    ],
    [
      "44.99136154",
      "24.842828325"
    ],
    [
      "12",
      "12"
    ],
    [
      "13",
      "42.699275549"
    ]
  ],
  "side": "55.99136154"
}

Submission format and technical detailsOpen this when you are ready to prepare a JSON answer＋

Instance parameters

{
  "n": 13
}

Starter answer

{
  "centers": [
    [
      "42.618729721",
      "12.546346412"
    ],
    [
      "23.432749791",
      "53.968925233"
    ],
    [
      "14.932881594",
      "26.792121959"
    ],
    [
      "4.146767156",
      "26.75153379"
    ],
    [
      "32.282868199",
      "38.694808225"
    ],
    [
      "30.544956025",
      "49.99136154"
    ],
    [
      "48.939187002",
      "7.052174538"
    ],
    [
      "26.042855313",
      "26.240723811"
    ],
    [
      "32.784609691",
      "9"
    ],
    [
      "45.99136154",
      "45.819005289"
    ],
    [
      "44.99136154",
      "24.842828325"
    ],
    [
      "12",
      "12"
    ],
    [
      "13",
      "42.699275549"
    ]
  ],
  "side": "55.99136154"
}

Submit the square side length and the circle centers listed in order of radii 1, 2, …, n. Write each number as a decimal string such as "3.5". · Verifier v1.0.0

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