nd4j-common/src/main/java/org/nd4j/linalg/util/BigDecimalMath.java
static public BigDecimal tan(final BigDecimal x) {
        if (x.compareTo(BigDecimal.ZERO) == 0) {
            return BigDecimal.ZERO;
        } else if (x.compareTo(BigDecimal.ZERO) < 0) {
            return tan(x.negate()).negate();
        } else {
            /* reduce modulo pi
             */
            BigDecimal res = modpi(x);
            /* absolute error in the result is err(x)/cos^2(x) to lowest order
             */
            final double xDbl = res.doubleValue();
            final double xUlpDbl = x.ulp().doubleValue() / 2.;
            final double eps = xUlpDbl / 2. / Math.pow(Math.cos(xDbl), 2.);
            if (xDbl > 0.8) {
                /* tan(x) = 1/cot(x) */
                BigDecimal co = cot(x);
                MathContext mc = new MathContext(err2prec(1. / co.doubleValue(), eps));
                return BigDecimal.ONE.divide(co, mc);
            } else {
                final BigDecimal xhighpr = scalePrec(res, 2);
                final BigDecimal xhighprSq = multiplyRound(xhighpr, xhighpr);
                BigDecimal result = xhighpr.plus();
                /* x^(2i+1) */
                BigDecimal xpowi = xhighpr;
                Bernoulli b = new Bernoulli();
                /* 2^(2i) */
                BigInteger fourn = BigInteger.valueOf(4);
                /* (2i)! */
                BigInteger fac = BigInteger.valueOf(2);
                for (int i = 2;; i++) {
                    Rational f = b.at(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);
                    result = result.add(c);
                    if (Math.abs(c.doubleValue()) < 0.1 * eps) {
                        break;
                    }
                }
                MathContext mc = new MathContext(err2prec(result.doubleValue(), eps));
                return result.round(mc);
            }
        }
    }
static public BigDecimal tan(final BigDecimal x) {
        if (x.compareTo(BigDecimal.ZERO) == 0) {
            return BigDecimal.ZERO;
        } else if (x.compareTo(BigDecimal.ZERO) < 0) {
            return tan(x.negate()).negate();
        } else {
            /* reduce modulo pi
             */
            BigDecimal res = modpi(x);
            /* absolute error in the result is err(x)/cos^2(x) to lowest order
             */
            final double xDbl = res.doubleValue();
            final double xUlpDbl = x.ulp().doubleValue() / 2.;
            final double eps = xUlpDbl / 2. / Math.pow(Math.cos(xDbl), 2.);
            if (xDbl > 0.8) {
                /* tan(x) = 1/cot(x) */
                BigDecimal co = cot(x);
                MathContext mc = new MathContext(err2prec(1. / co.doubleValue(), eps));
                return BigDecimal.ONE.divide(co, mc);
            } else {
                final BigDecimal xhighpr = scalePrec(res, 2);
                final BigDecimal xhighprSq = multiplyRound(xhighpr, xhighpr);
                BigDecimal result = xhighpr.plus();
                /* x^(2i+1) */
                BigDecimal xpowi = xhighpr;
                Bernoulli b = new Bernoulli();
                /* 2^(2i) */
                BigInteger fourn = BigInteger.valueOf(4);
                /* (2i)! */
                BigInteger fac = BigInteger.valueOf(2);
                for (int i = 2;; i++) {
                    Rational f = b.at(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);
                    result = result.add(c);
                    if (Math.abs(c.doubleValue()) < 0.1 * eps) {
                        break;
                    }
                }
                MathContext mc = new MathContext(err2prec(result.doubleValue(), eps));
                return result.round(mc);
            }
        }
    }
static public BigDecimal tan ( final BigDecimal x ) { if ( x . compareTo ( BigDecimal . ZERO ) == 0 ) { return BigDecimal . ZERO ; } else if ( x . compareTo ( BigDecimal . ZERO ) < 0 ) { return tan ( x . negate ( ) ) . negate ( ) ; } else { /* reduce modulo pi
             */ BigDecimal res = modpi ( x ) ; /* absolute error in the result is err(x)/cos^2(x) to lowest order
             */ final double xDbl = res . doubleValue ( ) ; final double xUlpDbl = x . ulp ( ) . doubleValue ( ) / 2. ; final double eps = xUlpDbl / 2. / Math . pow ( Math . cos ( xDbl ) , 2. ) ; if ( xDbl > 0.8 ) { /* tan(x) = 1/cot(x) */ BigDecimal co = cot ( x ) ; MathContext mc = new MathContext ( err2prec ( 1. / co . doubleValue ( ) , eps ) ) ; return BigDecimal . ONE . divide ( co , mc ) ; } else { final BigDecimal xhighpr = scalePrec ( res , 2 ) ; final BigDecimal xhighprSq = multiplyRound ( xhighpr , xhighpr ) ; BigDecimal result = xhighpr . plus ( ) ; /* x^(2i+1) */ BigDecimal xpowi = xhighpr ; Bernoulli b = new Bernoulli ( ) ; /* 2^(2i) */ BigInteger fourn = BigInteger . valueOf ( 4 ) ; /* (2i)! */ BigInteger fac = BigInteger . valueOf ( 2 ) ; for ( int i = 2 ; ; i ++ ) { Rational f = b . at ( 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 ) ; result = result . add ( c ) ; if ( Math . abs ( c . doubleValue ( ) ) < 0.1 * eps ) { break ; } } MathContext mc = new MathContext ( err2prec ( result . doubleValue ( ) , eps ) ) ; return result . round ( mc ) ; } } }
The trigonometric tangent.

@param x the argument in radians.
@return the tan(x)
The trigonometric tangent .