A006879 Number of Primes by Number of Digits

```using System.Collections.Generic;
using System.Numerics;

namespace Algorithms.Sequences
{
/// <summary>
///     <para>
///         Number of primes with n digits
///         (The number of primes between 10^(n-1) and 10^n).
///     </para>
///     <para>
///         Wikipedia: https://wikipedia.org/wiki/Prime-counting_function.
///     </para>
///     <para>
///         OEIS: https://oeis.org/A006879.
///     </para>
/// </summary>
public class NumberOfPrimesByNumberOfDigitsSequence : ISequence
{
/// <summary>
/// Gets sequence of number of primes.
/// </summary>
public IEnumerable<BigInteger> Sequence
{
get
{
ISequence primes = new PrimesSequence();
var powerOf10 = new BigInteger(1);
var counter = new BigInteger(0);

foreach (var p in primes.Sequence)
{
if (p > powerOf10)
{
yield return counter;
counter = 0;
powerOf10 *= 10;
}

counter++;
}
}
}
}
}
```