Problem 72

Problem 72 - Project Euler
40秒。

phi <- 1:10^6
for(i in 2:10^6){
  if(phi[i]==i){                        #i is prime
    for(j in seq(i,10^6,i)){            #j has i as prime factor
      phi[j] <- phi[j]*(i-1)/i          #and j is not prime
    }
  }
}
sum(phi)-1