Calculating the number of offspring for field voles.

Voles.cs

public class Voles 
{
  private static int _daysBeforeFirst = 25;
  private static int _daysBetween = 20;
 
  private static Dictionary<int, long> _cache = 
    new Dictionary<int, long>();
  
  public static long F(int days) {
    if (!_cache.ContainsKey(days)) {
      _cache[days] = F0(days);
    }
    return _cache[days];
  }

  private static long F0(int days) {
    int end = days - _daysBeforeFirst;
    if (end < 0) {
      return 1;
    }
    int start = end % _daysBetween;
    long count = 0;
    for (int d = start; d <= end; d += _daysBetween) {
      count += F(d) + 1;
    }
    return 1 + 4 * count;
  }
}