Task: Chowla numbers
Go: package main

import "fmt"

func chowla(n int) int {
    if n < 1 {
        panic("argument must be a positive integer")
    }
    sum := 0
    for i := 2; i*i <= n; i++ {
        if n%i == 0 {
            j := n / i
            if i == j {
                sum += i
            } else {
                sum += i + j
            }
        }
    }
    return sum
}

func sieve(limit int) []bool {
    
    
    c := make([]bool, limit)
    for i := 3; i*3 < limit; i += 2 {
        if !c[i] && chowla(i) == 0 {
            for j := 3 * i; j < limit; j += 2 * i {
                c[j] = true
            }
        }
    }
    return c
}

func commatize(n int) string {
    s := fmt.Sprintf("%d", n)
    le := len(s)
    for i := le - 3; i >= 1; i -= 3 {
        s = s[0:i] + "," + s[i:]
    }
    return s
}

func main() {
    for i := 1; i <= 37; i++ {
        fmt.Printf("chowla(%2d) = %d\n", i, chowla(i))
    }
    fmt.Println()

    count := 1
    limit := int(1e7)
    c := sieve(limit)
    power := 100
    for i := 3; i < limit; i += 2 {
        if !c[i] {
            count++
        }
        if i == power-1 {
            fmt.Printf("Count of primes up to %-10s = %s\n", commatize(power), commatize(count))
            power *= 10
        }
    }

    fmt.Println()
    count = 0
    limit = 35000000
    for i := uint(2); ; i++ {
        p := 1 << (i - 1) * (1<<i - 1) 
        if p > limit {
            break
        }
        if chowla(p) == p-1 {
            fmt.Printf("%s is a perfect number\n", commatize(p))
            count++
        }
    }
    fmt.Println("There are", count, "perfect numbers <= 35,000,000")
}

Java: public class Chowla {

    public static void main(String[] args) {
        int[] chowlaNumbers = findChowlaNumbers(37);
        for (int i = 0; i < chowlaNumbers.length; i++) {
            System.out.printf("chowla(%d) = %d%n", (i+1), chowlaNumbers[i]);
        }
        System.out.println();

        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++) {
            if (chowla(n) == 0) count++;
            if (n % power == 0) {
                num[i][0] = power;
                num[i][1] = count;
                i++;
                power *= 10;
            }
        }
        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];

        int k = 2, kk = 3, p;
        while ((p = k * kk) < limit) {
            if (chowla(p) == p - 1) {
                num = increaseArr(num);
                num[count++] = p;
            }
            k = kk + 1;
            kk += k;
        }
        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;
    }
}