Problem 44

Problem 44 - Project Euler
これはひどい
もう少し考える．
＠01:50 無理．もうあきらめようか．

pent <- function(n) n*(3*n-1)/2
is.pentagonal <- function(p){
  n <- ceiling(sqrt(2/3*p))
  2*p == n*(3*n-1)
}

j <- 2
ans <- 0
while(ans==0){
  for(k in 1:(j-1)){
    if(is.pentagonal(pent(j)+pent(k)) &&
       is.pentagonal(pent(j)-pent(k))){
      ans <- pent(j)-pent(k)
    }
  }
  j <- j+1
}