Problem Link :http://www.usaco.org/index.php?page=viewproblem2&cpid=1043 My approach was let we will store the K in vector for Each i [1...N] now let say n+1 can be represented as sum of x+y i:e n+1= x+y so answer will be set[n]=Lcm of (Set[x],Set[y]) for every unique pair of x,y but this solution is not working can anyone give correct solution
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The way I solved it: You can divide the permutation in cycles, and the LCM of the cycle lengths of a certain permutation will be a valid K. It can be shown that all K that can be achieved for N cows, can be made of only cycles of length $$$p^k$$$ where p is a prime number, and cycles of length 1. With this observation you can make a DP state of
$$$DP[n][q] = $$$ sum of all valid K's of all permutations of length n where you only used cycles of 1, and $$$p^k$$$ for all primes $$$p \leq q$$$.
The recurrence will then be: $$$DP[n][q] = DP[n][p] + \sum DP[n-q^k][p] \cdot q^k$$$, where p is the previous prime number.
Didnt understood this line :
as for Ex n =5 how is LCM 6 is achieved
If you have cycles of $$$2^1$$$ and $$$3^1$$$, then you get that the LCM of the lengths of cycles is 6, and the sum of cycles is n=5. Also for the DP, to account for cycles of length 1, you set $$$DP[j][1] = 1$$$ for all $$$0 \leq j \leq n$$$.