EXP(3) BSD Programmer's Manual EXP(3)[top]NAMEexp, expm1, log, log10, log1p, pow - exponential, logarithm, power func- tionsSYNOPSIS#include <math.h> double exp(double x); double expm1(double x); double log(double x); double log10(double x); double log1p(double x); double pow(double x, double y);DESCRIPTIONThe exp() function computes the exponential value of the given argument x. The expm1() function computes the value exp(x)-1 accurately even for tiny argument x. The log() function computes the value for the natural logarithm of the argument x. The log10() function computes the value for the logarithm of argument x to base 10. The log1p() function computes the value of log(1+x) accurately even for tiny argument x. The pow() computes the value of x to the exponent y.ERROR (due to Roundoff etc.)exp(x), log(x), expm1(x) and log1p(x) are accurate to within an up, and log10(x) to within about 2 ups; an up is one Unit in the Last Place. The error in pow(x, y) is below about 2 ups when its magnitude is moderate, but increases as pow(x, y) approaches the over/underflow thresholds until almost as many bits could be lost as are occupied by the floating-point format's exponent field; that is 8 bits for VAX D and 11 bits for IEEE 754 Double. No such drastic loss has been exposed by testing; the worst errors observed have been below 20 ups for VAX D, 300 ups for IEEE 754 Double. Moderate values of pow() are accurate enough that pow(integer, integer) is exact until it is bigger than 2**56 on a VAX, 2**53 for IEEE 754.RETURN VALUESThese functions will return the appropriate computation unless an error occurs or an argument is out of range. The functions exp(), expm1() and pow() detect if the computed value will overflow, set the global variable errno to RANGE and cause a reserved operand fault on a VAX or Tahoe. The function pow(x, y) checks to see if x < 0 and y is not an integer, in the event this is true, the global variable errno is set to EDOM and on the VAX and Tahoe generate a reserved operand fault. On a VAX and Tahoe, errno is set to EDOM and the reserved operand is returned by log unless x > 0, by log1p() unless x >-1.NOTESThe functions exp(x)-1 and log(1+x) are calledexpm1and logp1 in BASIC on the Hewlett-Packard HP-71B and APPLE Macintosh, EXP1 and LN1 in Pas- cal, exp1 and log1 in C on APPLE Macintoshes, where they have been pro- vided to make sure financial calculations of ((1+x)**n-1)/x, namely expm1(n*log1p(x))/x, will be accurate when x is tiny. They also provide accurate inverse hyperbolic functions. The function pow(x, 0) returns x**0 = 1 for all x including x = 0, Infin- ity (not found on a VAX), and NaN (the reserved operand on a VAX). Previous implementations of pow may have defined x**0 to be undefined in some or all of these cases. Here are reasons for returning x**0 = 1 al- ways: 1. Any program that already tests whether x is zero (or infinite or NaN) before computing x**0 cannot care whether 0**0 = 1 or not. Any program that depends upon 0**0 to be invalid is dubious any- way since that expression's meaning and, if invalid, its conse- quences vary from one computer system to another. 2. Some Algebra texts (e.g. Sigler's) define x**0 = 1 for all x, in- cluding x = 0. This is compatible with the convention that ac- cepts a[0] as the value of polynomial p(x) = a[0]*x**0 + a[1]*x**1 + a[2]*x**2 +...+ a[n]*x**n at x = 0 rather than reject a[0]*0**0 as invalid. 3. Analysts will accept 0**0 = 1 despite that x**y can approach any- thing or nothing as x and y approach 0 independently. The reason for setting 0**0 = 1 anyway is this: If x(z) and y(z) are any functions analytic (expandable in power series) in z around z = 0, and if there x(0) = y(0) = 0, then x(z)**y(z) -> 1 as z -> 0. 4. If 0**0 = 1, then infinity**0 = 1/0**0 = 1 too; and then NaN**0 = 1 too because x**0 = 1 for all finite and infinite x, i.e., inde- pendently of x.SEE ALSOmath(3), infnan(3)HISTORYA exp(), log() and pow() function appeared in Version 6 AT&T UNIX. A log10() function appeared in Version 7 AT&T UNIX. The log1p() and expm1() functions appeared in 4.3BSD.4th Berkeley DistributionApril 19, 1994 2

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