### 4` `Mathematical Functions

This chapter describes the basic mathematical constants and functions provided by the PLT Scheme Science Collection.

The constants and functions described in this chapter are defined in the "math.ss" file in the science collection and are made available using the form:

(require (planet williams/science/math)) |

#### 4.1` `Mathematical Constants

The following table shows the mathematical constants defined by the PLT Scheme Science Collection:

e | The base of the exponentials, |

log2e | The base two logarithm of |

log10e | The base ten logarithm of |

sqrt2 | The square root of two, |

sqrt1/2 | The square root of one half, |

sqrt3 | The square root of three, |

pi | The constant pi, π |

pi/2 | Pi divided by two, π |

pi/4 | Pi divided by four, π |

sqrtpi | The square root of pi, |

2/sqrtpi | Two divided by the square root of pi, 2 |

1/pi | The reciprocal of pi, 1 |

2/pi | Twice the reciprocal of pi, 2 |

ln10 | The natural log of ten, |

ln2 | The natural log of two, |

lnpi | The natural log of pi, |

euler | Euler’s constant, |

#### 4.2` `Testing for Infinities and Not-a-Number

PLT Scheme provides +inf.0 (positive infinity), -inf.0 (negative infinity), +nan.0 (not a number), and +nan.0 (same as +nan.0) as inexact numerical constants. The following functions are provided as a convenience for checking for infinities and not-a-number.

x : any/c |

Returns true, #t, if x is not-a-number (i.e., equivalent to +nan.0 or, equivalently, +nan.0), and false, #f, otherwise. Note that this is not the same as (not (number? x)), which is true if x is not a number.

x : any/c |

Returns 1 if x is positive infinity (i.e., equivalent to +inf.0), -1 if x is negative infinity (i.e., equivalent to -inf.0), and false, #f, otherwise. Note that (finite? x) is not equivalent to (not (infinite? x)), since both finite? and infinite? return false, #f for anything that is not a real number.

x : any/c |

Returns true, #t, if x is a finite real number and false, #f, otherwise. Note that (finite? x) is not equivalent to (not (infinite? x)), since both finite? and infinite? return false, #f for anything that is not a real number.

#### 4.3` `Elementary Functions

The following functions provide some elementary mathematical functions that are not provide by PLT Scheme.

x : real? |

Computes the value of *log*(1* + x*) in a way that is accurate for small x.

x : real? |

Computes the value of *exp*(*x - *1) in a way that is accurate for small x.

x : real? |

y : real? |

Computes the value of (*x*^{2}* + y*^{2})^{½} in a way that avoids overflow.

x : real? |

Computes the value of the hyperbolic arccosine, *arccosh*, of x.

x : real? |

Computes the value of the hyperbolic arcsine, *arcsinh*, of x.

x : real? |

Computes the value of the hyperbolic arctangent, *arctanh*, of x.

x : real? |

e : integer? |

Computes the value of *x × *2^{e}.

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x : real? |

Splits the real number x into a normalized fraction *f* and exponent *e* such that *x = f × *2^{e} and 0*.*5* ≤ f < *1. The function returns *f* and *e* as multiple values. If x is zero, both *f* and *e* are returned as zero.

#### 4.4` `Testing the Sign of Numbers

x : real? |

Returns the sign of x as 1 if *x ≥ *0 and -1 if *x < *0. Note that the sign of zero is positive, regardless of its floating-point sign bit.

#### 4.5` `Approximate Comparisons of Real Numbers

It is sometimes useful to be able to compare two real (in particular, floating-point) numbers approximately to allow for rounding and truncation errors. The following functions implements the approximate floating-point comparison algorithm proposed by D.E. Knuth in Section 4.2.2 of *Seminumerical Algorithms* (3rd edition) [Knuth].

x : real? |

y : real? |

epsilon : real? |

Determines whether x and y are approximately equal to within a relative accuracy, epsilon. The relative accuracy is measured using an interval of 2* × delta*, where *delta = *2^{k}* × epsilon* and *k* is the maximum base 2 exponent of x and y as computed by the function frexp. If x and y lie within this interval, they are considered equal and the function returns 0. Otherwise, if *x < y*, the function returns -1, or if *x > y>*, the function returns 1.

The implementation of this function is based on the packege fcmp by T.C. Belding.