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id	title	challengeType	forumTopicId	dashedName
5900f3a61000cf542c50feb9	Problem 58: Spiral primes	5	302169	problem-58-spiral-primes

--description--

Starting with 1 and spiralling anticlockwise in the following way, a square spiral with side length 7 is formed.

37 36 35 34 33 32 31
38 17 16 15 14 13 30
39 18 5 4 3 12 29
40 19 6 1 2 11 28
41 20 7 8 9 10 27
42 21 22 23 24 25 26
43 44 45 46 47 48 49

It is interesting to note that the odd squares lie along the bottom right diagonal, but what is more interesting is that 8 out of the 13 numbers lying along both diagonals are prime; that is, a ratio of 8/13 ≈ 62%.

If one complete new layer is wrapped around the spiral above, a square spiral with side length 9 will be formed. If this process is continued, what is the side length of the square spiral for which the ratio of primes along both diagonals first falls below 10%?

--hints--

spiralPrimes() should return a number.

assert(typeof spiralPrimes() === 'number');

spiralPrimes() should return 26241.

assert.strictEqual(spiralPrimes(), 26241);

--seed--

--seed-contents--

function spiralPrimes() {

  return true;
}

spiralPrimes();

--solutions--

// solution required

1.7 KiB Raw Blame History