Chowla numbers: Difference between revisions

int[] chowlaNumbers = findChowlaNumbers(37);

for (int i = 0; i < chowlaNumbers.length; i++) {

System.out.printf("chowla(%d) = %d%n", (i+1), chowlaNumbers[i]);

int[][] primes = countPrimes(100, 10_000_000);

for (int i = 0; i < primes.length; i++) {

System.out.printf(Locale.US, "There is %,d primes up to %,d%n", primes[i][1], primes[i][0]);

}

System.out.println();

int[] perfectNumbers = findPerfectNumbers(35_000_000);

for (int i = 0; i < perfectNumbers.length; i++) {

System.out.printf("%d is a perfect number%n", perfectNumbers[i]);

}

System.out.printf(Locale.US, "There are %d perfect numbers < %,d%n", perfectNumbers.length, 35_000_000);

}

public static int chowla(int n) {

if (n < 0) throw new IllegalArgumentException("n is not positive");

int sum = 0;

for (int i = 2, j; i * i <= n; i++)

if (n % i == 0) sum += i + (i == (j = n / i) ? 0 : j);

return sum;

}

protected static int[][] countPrimes(int power, int limit) {

int count = 0;

int[][] num = new int[countMultiplicity(limit, power)][2];

for (int n = 2, i=0; n <= limit; n++) {

num[i][0] = power;

num[i][1] = count;

i++;

return num;

}

protected static int countMultiplicity(int limit, int start) {

int count = 0;

int cur = limit;

while(cur >= start) {

count++;

cur = cur/10;

}

return count;

}

protected static int[] findChowlaNumbers(int limit) {

int[] num = new int[limit];

for (int i = 0; i < limit; i++) {

num[i] = chowla(i+1);

}

return num;

}

protected static int[] findPerfectNumbers(int limit) {

int count = 0;

int[] num = new int[count];

num = increaseArr(num);

num[count++] = p;

return num;

private static int[] increaseArr(int[] arr) {

int[] tmp = new int[arr.length + 1];

System.arraycopy(arr, 0, tmp, 0, arr.length);

return tmp;