### {

I had no idea:

2011 = 157 + 163 + 167 + 173 + 179 + 181 + 191 + 193 + 197 + 199 + 211

After learning that not only was 2011 was prime, but that it was the sum of consecutive primes, I sought to write some C# code that would demonstrate this. I wrote it in a functional style, meaning at some point to port it to F# – you'll recognize my in a forthcoming post on computing primes with F# which is actually an implementation of the algorithm I first worked through in the below C#.

Func<long[], IEnumerable<KeyValuePair<long, string>>> GenerateConsecutivePrimeSums = (primes) =>

{

List<KeyValuePair<long, string>> primeSums = new List<KeyValuePair<long, string>>();

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

{

List<string> operands = new List<string>();

long primeSum = primes[i];

operands.Add(primeSum.ToString());

for (int j = i + 1; j < primes.Count(); j++)

{

primeSum += primes[j];

operands.Add(primes[j].ToString());

if (!primeSums.Any(ps => ps.Key == primeSum) && primes.Contains(primeSum))

{

primeSums.Add(new KeyValuePair<long, string>(primeSum, String.Join(" + ", operands.ToArray())));

}

}

}

return primeSums;

};

Action primesAndSums = () => {

// generate primes up to 2020

var primes = GeneratePrimes(2020);

// generate arrays of consecutive primes that add up to prime

var primesSummed = GenerateConsecutivePrimeSums(primes.OrderBy(a => a).ToArray());

// go through the summed primes (dictionary of prime as key, added numbers as value)

// Console.WriteLine formatted results

primesSummed

.OrderBy(ps => ps.Key)

.ToList()

.ForEach(ps => Console.WriteLine(ps.Key + " = " + ps.Value));

};

primesAndSums();

The code actually calculates sums of the primes it generates, click here for all the other primes which are sums of consecutive primes up to 2020: