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#### Problem

A perfect number is a number for which the sum of its proper divisors is exactly equal to the number. For example, the sum of the proper divisors of 28 would be 1 + 2 + 4 + 7 + 14 = 28, which means that 28 is a perfect number.

A numbernis called deficient if the sum of its proper divisors is less thannand it is called abundant if this sum exceedsn.

As 12 is the smallest abundant number, 1 + 2 + 3 + 4 + 6 = 16, the smallest number that can be written as the sum of two abundant numbers is 24. By mathematical analysis, it can be shown that all integers greater than 28123 can be written as the sum of two abundant numbers. However, this upper limit cannot be reduced any further by analysis even though it is known that the greatest number that cannot be expressed as the sum of two abundant numbers is less than this limit.

Find the sum of all the positive integers which cannot be written as the sum of two abundant numbers.

#### Solution

let findDivisors(n) = let upperBound = int32(sqrt(double(n))) [1..upperBound] |> Seq.filter (fun x -> n % x = 0) |> Seq.collect (fun x -> [x; n/x]) |> Seq.filter (fun x -> x <> n) |> Seq.distinct let isAbundantNumber(n) = (findDivisors(n) |> Seq.sum) > n let abundantNumbers = Seq.unfold (fun x -> if x < 28123 then Some(x, x+1) else None) 1 |> Seq.filter isAbundantNumber |> Seq.toList let abundantNumbersSums = abundantNumbers |> Seq.collect (fun n -> abundantNumbers |> List.map (fun m -> n + m)) |> Seq.filter (fun n -> n <= 28123) |> Seq.distinct |> Seq.toList let answer = ([1..28123] |> List.sum) - (abundantNumbersSums |> List.sum)

I had to make a small modification to the *findDivisors* function which I had used previously to make sure we don’t have any duplicates in there as we now need to sum the divisors.

I first defined the *isAbundantNumber* function to check if the sum of the divisors of a number n is greater than n itself as per the definition given in the brief.

Then I generated the list of ALL abundant numbers from 1 to 28122 because though 28123 is the upper limit, if it it’s to be the sum of two POSITIVE integers then the two integers must be in the range of 1 to 28122.

The next list *abundantNumbersSums* is the one that will take up the bulk of the 15 seconds it takes this code to run, it generates all the unique sums less or equal to 28123 that you can generate using two abundant numbers.

The answer is then achieved by finding the difference between a) the sum of all numbers from 1 to 28123, and b) the sum of the *abundantNumbersSum* list.

Hi, I’m **Yan**. I’m an **AWS Serverless Hero** and the author of **Production-Ready Serverless**.

I specialise in rapidly transitioning teams to serverless and building production-ready services on AWS.

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.

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.

Check out my new course, **Learn you some Lambda best practice for great good!** In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. **Enrol now and start learning**!

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Are you working with Serverless and looking for expert training to level-up your skills? Or are you looking for a solid foundation to start from? Look no further, register for my **Production-Ready Serverless workshop** to learn how to build production-grade Serverless applications!

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.

- 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?
- Many faced threats to Serverless security
- We can do better than percentile latencies
- Yubl’s road to Serverless
- AWS Lambda – should you have few monolithic functions or many single-purposed functions?
- AWS Lambda – compare coldstart time with different languages, memory and code sizes
- Guys, we’re doing pagination wrong
- Top 10 Serverless framework best practices