Problem 1096. Largest Twin Primes

Created by Des Mc Manus

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Twin primes are primes p1, p2 = p1 + 2 such that both p1 and p2 are prime numbers. Given a positive integer N (>=5) what are the largest twin primes (in order) <= N. For example, if N = 20 then p1 = 17 and p2 = 19.

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Last solution submitted on Oct 18, 2019

Last 200 Solutions

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Solution 551414

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1 Comment

Mattias on 1 Jan 2015

Was able to reduce size a few times and finally got to this solution by borrowing tricks used by Grzegorz Knor and Dan.

Solution 173238

2 Comments

2 Comments

Ryan on 5 Dec 2012

Tim,
Can you explain what the +'01'-'0' is doing? I don't see how i+'01'-'0'=[i,i+1].
Thanks

J.R.! Menzinger on 6 Dec 2012

'01 ' is a char vector of 2 positions: '0' ans '1'.
If you subtract '0' to it, it results a char vector with characters 0 and 1.
So '01'-'0' is the same as [0 1], but because of the '[]', it cost less operations.

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primes