Question

How to find the first integer equal to the sum of its digits raised to some power in C#

Answer 1

using System;

class DigitPower
{
    // compute sum of digits
    static long DigitSum(long n)
    {
        long s = 0;

        while (n > 0) {
            s += n % 10;
            n /= 10;
        }

        return s;
    }

    static void Main()
    {
        for (long n = 2; ; n++)
        {
            long s = DigitSum(n);

            // Try powers k = 2..10 (enough for reasonable ranges)
            long p = s * s;
            for (int k = 2; k <= 10; ++k) {
                if (p == n) {
                    Console.WriteLine($"Found: {n} = ({s})^{k}");
                    return;
                }
                p *= s; // next power
            }
        }
    }
}


/*
run:

Found: 81 = (9)^2

*/

 

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