with wave vectors 0 ≤ ≤ k 1 ≤ ≤ k 2 ≤ ≤ … … , ≤ ≤ k N , where the sum is over all N ! permutations, P , of the integers 1 , 2 , … … , N , and P maps 1 , 2 , … … , N to P 1 , P 2 , … … , P N . The coefficients a ( P ) , as well as the k 's are determined by the condition H ψ ψ = E ψ ψ , and this leads to a total energy