#### A006880 Number of Primes by Powers of 10

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

namespace Algorithms.Sequences
{
/// <summary>
///     <para>
///         Sequence of number of primes less than 10^n (with at most n digits).
///     </para>
///     <para>
///         Wikipedia: https://wikipedia.org/wiki/Prime-counting_function.
///     </para>
///     <para>
///         OEIS: https://oeis.org/A006880.
///     </para>
/// </summary>
public class NumberOfPrimesByPowersOf10Sequence : ISequence
{
/// <summary>
/// Gets sequence of numbers 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;
powerOf10 *= 10;
}

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