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PROJECT EULER · #0111

Primes with Runs

Statement only · SolvedOriginal problem ↗

Considering 4-digit primes containing repeated digits it is clear that they cannot all be the same: 1111 is divisible by 11, 2222 is divisible by 22, and so on. But there are nine 4-digit primes containing three ones: 1117,1151,1171,1181,1511,1811,2111,4111,8111.

We shall say that M(n,d) represents the maximum number of repeated digits for an n-digit prime where d is the repeated digit, N(n,d) represents the number of such primes, and S(n,d) represents the sum of these primes.

So M(4,1)=3 is the maximum number of repeated digits for a 4-digit prime where one is the repeated digit, there are N(4,1)=9 such primes, and the sum of these primes is S(4,1)=22275. It turns out that for d=0, it is only possible to have M(4,0)=2 repeated digits, but there are N(4,0)=13 such cases.

In the same way we obtain the following results for 4-digit primes.

Digit, d M(4, d) N(4, d) S(4, d)
0 2 13 67061
1 3 9 22275
2 3 1 2221
3 3 12 46214
4 3 2 8888
5 3 1 5557
6 3 1 6661
7 3 9 57863
8 3 1 8887
9 3 7 48073

For d=0 to 9, the sum of all S(4,d) is 273700.

Find the sum of all S(10,d).

Write-up coming later

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