Add A000215 Fermat Numbers sequence (TheAlgorithms#298)

Author: OleksiiHe

Algorithms.Tests/Sequences/FermatNumbersSequenceTests.cs

@@ -0,0 +1,19 @@
+using System.Linq;
+using System.Numerics;
+using Algorithms.Sequences;
+using FluentAssertions;
+using NUnit.Framework;
+
+namespace Algorithms.Tests.Sequences
+{
+    public class FermatNumbersSequenceTests
+    {
+        [Test]
+        public void First5ElementsCorrect()
+        {
+            var sequence = new FermatNumbersSequence().Sequence.Take(5);
+            sequence.SequenceEqual(new BigInteger[] { 3, 5, 17, 257, 65537 })
+                .Should().BeTrue();
+        }
+    }
+}

Algorithms/Sequences/FermatNumbersSequence.cs

@@ -0,0 +1,36 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace Algorithms.Sequences
+{
+    /// <summary>
+    ///     <para>
+    ///         Sequence of Fermat numbers: a(n) = 2^(2^n) + 1.
+    ///     </para>
+    ///     <para>
+    ///         Wikipedia: https://wikipedia.org/wiki/Fermat_number.
+    ///     </para>
+    ///     <para>
+    ///         OEIS: https://oeis.org/A000215.
+    ///     </para>
+    /// </summary>
+    public class FermatNumbersSequence : ISequence
+    {
+        /// <summary>
+        /// Gets sequence of Fermat numbers.
+        /// </summary>
+        public IEnumerable<BigInteger> Sequence
+        {
+            get
+            {
+                var n = new BigInteger(2);
+
+                while (true)
+                {
+                    yield return n + 1;
+                    n *= n;
+                }
+            }
+        }
+    }
+}

README.md

@@ -104,6 +104,7 @@ This repository contains algorithms and data structures implemented in C# for ed
     * [A000079 Powers of 2](./Algorithms/Sequences/PowersOf2Sequence.cs)
     * [A000108 Catalan](./Algorithms/Sequences/CatalanSequence.cs)
     * [A000142 Factorial](./Algorithms/Sequences/FactorialSequence.cs)
+    * [A000215 Fermat Numbers](./Algorithms/Sequences/FermatNumbersSequence.cs)
     * [A000290 Squares](./Algorithms/Sequences/SquaresSequence.cs)
     * [A000578 Cubes](./Algorithms/Sequences/CubesSequence.cs)
     * [A000720 PrimePi](./Algorithms/Sequences/PrimePiSequence.cs)