My Math Forum Prime Numbers Sum Algorithm Question

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 November 30th, 2011, 07:23 AM #1 Newbie   Joined: Nov 2011 Posts: 1 Thanks: 0 Prime Numbers Sum Algorithm Question Hello guys and girls i am writting for the first time in this forum.To my question-> We have a number which we know is a sum of prime numbers. Lets say the number is 64. it can be represented as two sum : 2+3+5+7+11+13+23 = 64 or 2+3+5+7+11+17+19 = 64 My question is if there is a way to find the combinations of which prime numbers summed is the number.A little confusing... In my example if we have 64 is there a way to find to find those 2 combinations listed and all possible ones for bigger numbers. Again we know that the number is a sum of prime numbers we just need to find all combinations of which.And one more thing the prime numbers cannot be repeated this is very important. Thank you in advance.
 November 30th, 2011, 08:53 AM #2 Global Moderator     Joined: Nov 2006 From: UTC -5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms Re: Prime Numbers Sum Algorithm Question This is a hard problem, since there may be a large number of ways. The problem can be solved by recursion: Code: ways(n,lim=n)=if(n<2||lim<2,return(n==0));my(s);forprime(p=2,min(n,lim),s+=ways(n-p,p-1));s; ways(64) This sequence is Sloane's A000586; see that page for a table of values up to 1000. My simple function is only practical for n up to a few hundred; memoization would make it faster.

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