public int GetPeriodsOfSquareRoot(int N)
        {
            int a0 = GetClosestIntegertToSquare(N);

            if (a0 * a0 == N)
            {
                return(0);
            }

            List <PeriodParameters> periodParameterList = new List <PeriodParameters>();

            var periodParameters = new PeriodParameters {
                an = a0, sub = -a0, f = 1.0
            };

            periodParameterList.Add(periodParameters);

            while (periodParameterList.Count < 300)
            {
                periodParameters = CalcNextPeriodParameters(a0, N, periodParameters);
                periodParameterList.Add(periodParameters);

                var treff = periodParameterList.Where(r => r.an == periodParameters.an && r.sub == periodParameters.sub && Math.Abs(r.f - periodParameters.f) < 0.10).ToList();

                if (treff.Count() == 2)
                {
                    int index  = periodParameterList.IndexOf(treff[0]);
                    int index2 = periodParameterList.Count() - 1;

                    return(index2 - index);
                }
            }
            throw new Exception($"Periode ikke funnet for {N}");
        }
        public PeriodParameters CalcNextPeriodParameters(int a0, int N, PeriodParameters tt)
        {
            int f2 = N - tt.sub * tt.sub;

            int an = 0;

            while (-((an + 1) * f2 / tt.f) + (-1) * tt.sub >= -a0)
            {
                an++;
            }

            int sub = (int)-((an) * f2 / tt.f) + (-1) * tt.sub;

            var nextPeriodParameters = new PeriodParameters {
                an = an, sub = sub, f = (f2 / tt.f)
            };

            return(nextPeriodParameters);
        }