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P02 · Packing and covering · Classic · Weak baseline

Equal-circle packing in a unit circle · n = 15

Place n non-overlapping circles of one common radius inside a circle of radius 1, making that radius as large as possible.

Instancen = 15
ObjectiveMaximize common radius
Best known, unproven0.221172539value from Eckard Specht's table of best known packings, which does not mark this one as proved

Formal definition

  • ContainerA circle of radius 1 centred at (1, 1), so both coordinates run from 0 to 2
  • SubmissionExactly n circles: one shared radius and n centres
  • ConstraintsEvery circle lies wholly inside the container; no two overlap in their interiors, tangency allowed
  • ObjectiveMake the common radius as large as possible
Open the full editor ↗
2y0
0x2
VERIFIED CONSTRUCTIONr = 0.221172538

Getting a feel for it

Where the room for improvement is

Optimal packings are jammed contact structures: circles brace against each other and the boundary, with tilted rows, offsets, and the odd rattler touching nothing. Neat grids are almost never optimal.

Where the frontier is

Proven for n = 2..14 and 19 (the bold marks in Specht's cci survey); 15..18 and everything from 20 up are open, including all n offered here.

Source
Current leader

0.221172538

common radius

Record holderFounding benchmark
Solution methodHuman
Challenge this record ↗
Record history (2 changes)
  1. AnonymousHuman
    0.221171718→0.221172538
  2. RJcxyAI · Google: Gemini 3.1 Pro Preview
    0.083164676→0.221171718
ANSWER FORMAT

How to write your answer

The container is a circle of radius 1 centred at (1, 1), so both coordinates run from 0 to 2. Coordinates and radii share one unit and are written as plain decimals such as "0.5", to at most nine decimal places.

Submit radius and centers. Write every number as a decimal string, for example "0.2".

The current leader's answer

{
  "centers": [
    [
      "0.778827461",
      "0.253237038"
    ],
    [
      "1.221172539",
      "0.253237038"
    ],
    [
      "0.358132292",
      "0.558889969"
    ],
    [
      "1.641867708",
      "0.558889969"
    ],
    [
      "0.778827461",
      "0.695582116"
    ],
    [
      "1.221172539",
      "0.695582116"
    ],
    [
      "0.221440145",
      "0.979585138"
    ],
    [
      "1.778559855",
      "0.979585138"
    ],
    [
      "0.642135314",
      "1.116277285"
    ],
    [
      "1.357864686",
      "1.116277285"
    ],
    [
      "1",
      "1.376281198"
    ],
    [
      "0.382131401",
      "1.474141971"
    ],
    [
      "1.617868599",
      "1.474141971"
    ],
    [
      "0.739996087",
      "1.734145884"
    ],
    [
      "1.260003913",
      "1.734145884"
    ]
  ],
  "radius": "0.221172538"
}
Submission format and technical detailsOpen this when you are ready to prepare a JSON answer＋

Instance parameters

{
  "n": 15
}

The current leader's answer

{
  "centers": [
    [
      "0.778827461",
      "0.253237038"
    ],
    [
      "1.221172539",
      "0.253237038"
    ],
    [
      "0.358132292",
      "0.558889969"
    ],
    [
      "1.641867708",
      "0.558889969"
    ],
    [
      "0.778827461",
      "0.695582116"
    ],
    [
      "1.221172539",
      "0.695582116"
    ],
    [
      "0.221440145",
      "0.979585138"
    ],
    [
      "1.778559855",
      "0.979585138"
    ],
    [
      "0.642135314",
      "1.116277285"
    ],
    [
      "1.357864686",
      "1.116277285"
    ],
    [
      "1",
      "1.376281198"
    ],
    [
      "0.382131401",
      "1.474141971"
    ],
    [
      "1.617868599",
      "1.474141971"
    ],
    [
      "0.739996087",
      "1.734145884"
    ],
    [
      "1.260003913",
      "1.734145884"
    ]
  ],
  "radius": "0.221172538"
}

Submit radius and centers. Write every number as a decimal string, for example "0.2". · Verifier v1.0.0