public static BigDecimal Zeta(int n, MathContext mc)
{
if (n <= 0)
throw new NotSupportedException("Zeta at negative argument " + n + " not supported");
if (n == 1)
throw new ArithmeticException("Pole at zeta(1) ");
if (n%2 == 0) {
/* Even indices. Abramowitz-Stegun 23.2.16. Start with 2^(n-1)*B(n)/n!
*/
Rational b = Bernoulli.Default[n].Abs();
b = b.Divide(Factorial.Default[n]);
b = b.Multiply(BigInteger.One.ShiftLeft(n - 1));
/* to be multiplied by pi^n. Absolute error in the result of pi^n is n times
* error in pi times pi^(n-1). Relative error is n*error(pi)/pi, requested by mc.
* Need one more digit in pi if n=10, two digits if n=100 etc, and add one extra digit.
*/
var mcpi = new MathContext(mc.Precision + (int) (System.Math.Log10(10.0*n)));
BigDecimal piton = PiRound(mcpi).Pow(n, mc);
return MultiplyRound(piton, b);
}
if (n == 3) {
/* Broadhurst BBP <a href="http://arxiv.org/abs/math/9803067">arXiv:math/9803067</a>
* Error propagation: S31 is roughly 0.087, S33 roughly 0.131
*/
int[] a31 = {1, -7, -1, 10, -1, -7, 1, 0};
int[] a33 = {1, 1, -1, -2, -1, 1, 1, 0};
BigDecimal S31 = BroadhurstBbp(3, 1, a31, mc);
BigDecimal S33 = BroadhurstBbp(3, 3, a33, mc);
S31 = S31.Multiply(new BigDecimal(48));
S33 = S33.Multiply(new BigDecimal(32));
return S31.Add(S33).Divide(new BigDecimal(7), mc);
}
if (n == 5) {
/* Broadhurst BBP <a href=http://arxiv.org/abs/math/9803067">arXiv:math/9803067</a>
* Error propagation: S51 is roughly -11.15, S53 roughly 22.165, S55 is roughly 0.031
* 9*2048*S51/6265 = -3.28. 7*2038*S53/61651= 5.07. 738*2048*S55/61651= 0.747.
* The result is of the order 1.03, so we add 2 digits to S51 and S52 and one digit to S55.
*/
int[] a51 = {31, -1614, -31, -6212, -31, -1614, 31, 74552};
int[] a53 = {173, 284, -173, -457, -173, 284, 173, -111};
int[] a55 = {1, 0, -1, -1, -1, 0, 1, 1};
BigDecimal S51 = BroadhurstBbp(5, 1, a51, new MathContext(2 + mc.Precision));
BigDecimal S53 = BroadhurstBbp(5, 3, a53, new MathContext(2 + mc.Precision));
BigDecimal S55 = BroadhurstBbp(5, 5, a55, new MathContext(1 + mc.Precision));
S51 = S51.Multiply(new BigDecimal(18432));
S53 = S53.Multiply(new BigDecimal(14336));
S55 = S55.Multiply(new BigDecimal(1511424));
return S51.Add(S53).Subtract(S55).Divide(new BigDecimal(62651), mc);
}
/* Cohen et al Exp Math 1 (1) (1992) 25
*/
var betsum = new Rational();
var bern = new Bernoulli();
var fact = new Factorial();
for (int npr = 0; npr <= (n + 1)/2; npr++) {
Rational b = bern[2*npr].Multiply(bern[n + 1 - 2*npr]);
b = b.Divide(fact[2*npr]).Divide(fact[n + 1 - 2*npr]);
b = b.Multiply(1 - 2*npr);
if (npr%2 == 0)
betsum = betsum.Add(b);
else
betsum = betsum.Subtract(b);
}
betsum = betsum.Divide(n - 1);
/* The first term, including the facor (2pi)^n, is essentially most
* of the result, near one. The second term below is roughly in the range 0.003 to 0.009.
* So the precision here is matching the precisionn requested by mc, and the precision
* requested for 2*pi is in absolute terms adjusted.
*/
var mcloc = new MathContext(2 + mc.Precision + (int) (System.Math.Log10(n)));
BigDecimal ftrm = PiRound(mcloc).Multiply(new BigDecimal(2));
ftrm = ftrm.Pow(n);
ftrm = MultiplyRound(ftrm, betsum.ToBigDecimal(mcloc));
var exps = new BigDecimal(0);
/* the basic accuracy of the accumulated terms before multiplication with 2
*/
double eps = System.Math.Pow(10d, -mc.Precision);
if (n%4 == 3) {
/* since the argument n is at least 7 here, the drop
* of the terms is at rather constant pace at least 10^-3, for example
* 0.0018, 0.2e-7, 0.29e-11, 0.74e-15 etc for npr=1,2,3.... We want 2 times these terms
* fall below eps/10.
*/
int kmax = mc.Precision/3;
eps /= kmax;
/* need an error of eps for 2/(exp(2pi)-1) = 0.0037
* The absolute error is 4*exp(2pi)*err(pi)/(exp(2pi)-1)^2=0.0075*err(pi)
*/
BigDecimal exp2p = PiRound(new MathContext(3 + ErrorToPrecision(3.14, eps/0.0075)));
exp2p = Exp(exp2p.Multiply(new BigDecimal(2)));
BigDecimal c = exp2p.Subtract(BigDecimal.One);
exps = DivideRound(1, c);
for (int npr = 2; npr <= kmax; npr++) {
/* the error estimate above for npr=1 is the worst case of
* the absolute error created by an error in 2pi. So we can
* safely re-use the exp2p value computed above without
* reassessment of its error.
*/
c = PowRound(exp2p, npr).Subtract(BigDecimal.One);
c = MultiplyRound(c, (BigInteger.ValueOf(npr)).Pow(n));
c = DivideRound(1, c);
exps = exps.Add(c);
}
} else {
/* since the argument n is at least 9 here, the drop
* of the terms is at rather constant pace at least 10^-3, for example
* 0.0096, 0.5e-7, 0.3e-11, 0.6e-15 etc. We want these terms
* fall below eps/10.
*/
int kmax = (1 + mc.Precision)/3;
eps /= kmax;
/* need an error of eps for 2/(exp(2pi)-1)*(1+4*Pi/8/(1-exp(-2pi)) = 0.0096
* at k=7 or = 0.00766 at k=13 for example.
* The absolute error is 0.017*err(pi) at k=9, 0.013*err(pi) at k=13, 0.012 at k=17
*/
BigDecimal twop = PiRound(new MathContext(3 + ErrorToPrecision(3.14, eps/0.017)));
twop = twop.Multiply(new BigDecimal(2));
BigDecimal exp2p = Exp(twop);
BigDecimal c = exp2p.Subtract(BigDecimal.One);
exps = DivideRound(1, c);
c = BigDecimal.One.Subtract(DivideRound(1, exp2p));
c = DivideRound(twop, c).Multiply(new BigDecimal(2));
c = DivideRound(c, n - 1).Add(BigDecimal.One);
exps = MultiplyRound(exps, c);
for (int npr = 2; npr <= kmax; npr++) {
c = PowRound(exp2p, npr).Subtract(BigDecimal.One);
c = MultiplyRound(c, (BigInteger.ValueOf(npr)).Pow(n));
BigDecimal d = DivideRound(1, exp2p.Pow(npr));
d = BigDecimal.One.Subtract(d);
d = DivideRound(twop, d).Multiply(new BigDecimal(2*npr));
d = DivideRound(d, n - 1).Add(BigDecimal.One);
d = DivideRound(d, c);
exps = exps.Add(d);
}
}
exps = exps.Multiply(new BigDecimal(2));
return ftrm.Subtract(exps, mc);
}
public static BigDecimal Cot(BigDecimal x)
{
if (x.CompareTo(BigDecimal.Zero) == 0) {
throw new ArithmeticException("Cannot take cot of zero " + x);
}
if (x.CompareTo(BigDecimal.Zero) < 0) {
return Cot(x.Negate()).Negate();
}
/* reduce modulo pi
*/
BigDecimal res = ModPi(x);
/* absolute error in the result is err(x)/sin^2(x) to lowest order
*/
double xDbl = res.ToDouble();
double xUlpDbl = x.Ulp().ToDouble()/2d;
double eps = xUlpDbl/2d/System.Math.Pow(System.Math.Sin(xDbl), 2d);
BigDecimal xhighpr = ScalePrecision(res, 2);
BigDecimal xhighprSq = MultiplyRound(xhighpr, xhighpr);
var mc = new MathContext(ErrorToPrecision(xhighpr.ToDouble(), eps));
BigDecimal resul = BigDecimal.One.Divide(xhighpr, mc);
/* x^(2i-1) */
BigDecimal xpowi = xhighpr;
var b = new Bernoulli();
/* 2^(2i) */
var fourn = BigInteger.Parse("4");
/* (2i)! */
BigInteger fac = BigInteger.One;
for (int i = 1;; i++) {
Rational f = b[2*i];
fac = fac.Multiply(BigInteger.ValueOf((2*i))).Multiply(BigInteger.ValueOf((2*i - 1)));
f = f.Multiply(fourn).Divide(fac);
BigDecimal c = MultiplyRound(xpowi, f);
if (i%2 == 0)
resul = resul.Add(c);
else
resul = resul.Subtract(c);
if (System.Math.Abs(c.ToDouble()) < 0.1*eps)
break;
fourn = fourn.ShiftLeft(2);
xpowi = MultiplyRound(xpowi, xhighprSq);
}
mc = new MathContext(ErrorToPrecision(resul.ToDouble(), eps));
return resul.Round(mc);
}
public static BigDecimal Tan(BigDecimal x)
{
if (x.CompareTo(BigDecimal.Zero) == 0)
return BigDecimal.Zero;
if (x.CompareTo(BigDecimal.Zero) < 0) {
return Tan(x.Negate()).Negate();
}
/* reduce modulo pi
*/
BigDecimal res = ModPi(x);
/* absolute error in the result is err(x)/cos^2(x) to lowest order
*/
double xDbl = res.ToDouble();
double xUlpDbl = x.Ulp().ToDouble()/2d;
double eps = xUlpDbl/2d/System.Math.Pow(System.Math.Cos(xDbl), 2d);
if (xDbl > 0.8) {
/* tan(x) = 1/cot(x) */
BigDecimal co = Cot(x);
var mc = new MathContext(ErrorToPrecision(1d/co.ToDouble(), eps));
return BigDecimal.One.Divide(co, mc);
} else {
BigDecimal xhighpr = ScalePrecision(res, 2);
BigDecimal xhighprSq = MultiplyRound(xhighpr, xhighpr);
BigDecimal resul = xhighpr.Plus();
/* x^(2i+1) */
BigDecimal xpowi = xhighpr;
var b = new Bernoulli();
/* 2^(2i) */
BigInteger fourn = BigInteger.ValueOf(4);
/* (2i)! */
BigInteger fac = BigInteger.ValueOf(2);
for (int i = 2;; i++) {
Rational f = b[2*i].Abs();
fourn = fourn.ShiftLeft(2);
fac = fac.Multiply(BigInteger.ValueOf((2*i))).Multiply(BigInteger.ValueOf((2*i - 1)));
f = f.Multiply(fourn).Multiply(fourn.Subtract(BigInteger.One)).Divide(fac);
xpowi = MultiplyRound(xpowi, xhighprSq);
BigDecimal c = MultiplyRound(xpowi, f);
resul = resul.Add(c);
if (System.Math.Abs(c.ToDouble()) < 0.1*eps)
break;
}
var mc = new MathContext(ErrorToPrecision(resul.ToDouble(), eps));
return resul.Round(mc);
}
}