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#### Problem

Surprisingly there are only three numbers that can be written as the sum of fourth powers of their digits:

1634 = 1^{4}+ 6^{4}+ 3^{4}+ 4^{4}

8208 = 8^{4}+ 2^{4}+ 0^{4}+ 8^{4}

9474 = 9^{4}+ 4^{4}+ 7^{4}+ 4^{4}

As 1 = 1^{4}is not a sum it is not included.

The sum of these numbers is 1634 + 8208 + 9474 = 19316.

Find the sum of all the numbers that can be written as the sum of fifth powers of their digits.

#### Solution

// get the digits of a number into an array let getDigits (n:bigint) = n.ToString().ToCharArray() |> Array.map (fun c -> bigint.Parse(c.ToString())) // get the sum of a number's digits to the specified power let getDigitsToPowerSum (n:bigint) pow = getDigits(n) |> Array.map (fun x -> pown x pow) |> Array.sum // get the max sum achievable by a number of the given number of digits to the specified power let upperBound (digits:int) pow = bigint(digits) * pown 9I pow // find the last number of digits n, where the max sum achievable by the digits of a number of // n digits to the specified power is greater than the smallest number of n digits // any number with more than n digits do not need to be checked let digitsToCheck pow = let n = Seq.unfold (fun state -> Some(state, (state+1))) 1 |> Seq.filter (fun x -> (upperBound x pow).ToString().ToCharArray().Length < x) |> Seq.head n-1 // get the next number with the given number of digits let maxNumber digits = [1..digits] |> List.map (fun x -> 9I * pown 10I (x-1)) |> List.sum let answer = let max = maxNumber (digitsToCheck 5) [2I..max] |> List.filter (fun n -> n = getDigitsToPowerSum n 5) |> List.sum

One of the tricky things with this problem is that there’s no upper limit to how far you’d need to test before you can safely determine that you’ve tested all the numbers whose sum of fifth powers of their digits MIGHT be equal to the numbers themselves.

But that’s not to say it’s not possible, for a 1-digit number, i.e. 1..9, the max sum of its digits is 1 * (9 POW 5) = 59049; similarly for a 2-digit number the max sum is 2 * (9 POW 5) = 118098. The *upperBound* function calculates this upper ceiling for a number of a given digits and power.

Let’s take a closer look at the distribution of this upper limit as the number of digits go up, for the fourth power:

so as you can see, when the number of digits reaches 6, the max sum of fourth powers of digits is 39366, but the smallest 6-digit number is 100000! Therefore it’s impossible for any 6 digit number to be written as the sum of fourth powers of its digits, and it means we’ll need to go as far as covering all 5 digit numbers. The *digitsToCheck* function simply encapsulates this bit of logic:

The *maxNumber* function is a helper function which generates the biggest number of the given number of digits. To find the answer, I simply had to make use of all the helper functions mentioned so far and sum the numbers whose fifth powers of its digits equals itself.

Hi, I’m **Yan**. I’m an **AWS Serverless Hero** and I help companies go faster for less by adopting serverless technologies successfully.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

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We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.

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Check out my new podcast **Real-World Serverless** where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.

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Further reading

Here is a complete list of all my posts on serverless and AWS Lambda. In the meantime, here are a few of my most popular blog posts.

- All you need to know about caching for serverless applications
- Lambda optimization tip – enable HTTP keep-alive
- You are wrong about serverless and vendor lock-in
- You are thinking about serverless costs all wrong
- Just how expensive is the full AWS SDK?
- Check-list for going live with API Gateway and Lambda
- How to choose the right API Gateway auth method
- CloudFormation protip: use !Sub instead of !Join
- AWS Lambda – should you have few monolithic functions or many single-purposed functions?
- Guys, we’re doing pagination wrong
- Top 10 Serverless framework best practices
- How to break the “senior engineer” career ceiling
- My advice to junior developers