Bug #63136 [Opn->Nab]: pow() returns NaN where base is negative and exponent is an odd number root

Edit report at https://bugs.php.net/bug.php?id=63136&edit=1 ID: 63136 Updated by: nikic@php.net Reported by: tstobbe at sacbee dot com Summary: pow() returns NaN where base is negative and exponent is an odd number root -Status: Open +Status: Not a bug Type: Bug Package: Math related Operating System: Linux (fc17, 32-bit) PHP Version: 5.4.7 Block user comment: N Private report: N New Comment: Real-exponent powers are not defined for negative numbers, see http://en.wikipedia.org/wiki/Exponentiation#Real_exponents_with_negative_bases. You can see this very easily if you remember that a^b is defined as exp(b*ln(a)) and the logarithm is only defined in the range ]0,inf[. Previous Comments: ------------------------------------------------------------------------ [2012-09-22 02:47:21] tstobbe at sacbee dot com It should be noted that all subsequent fractional pow() issues are related to this. For example: pow(-[any number], 1/5) vs pow(-[any number], 2/5). Likewise I suspect that most other exponent based functions (such as log with an arbitrary base) are probably suspect, but this hasn't been verified. ------------------------------------------------------------------------ [2012-09-21 22:38:15] tstobbe at sacbee dot com Description: ------------ Odd roots (3, 5, etc) of negative numbers passed to pow() result in NaN rather than the actual result. While this makes sense for even numbered roots (sqrt and what-not), it wrecks havoc on any system that actually uses arbitrary roots of negative numbers. Example: -2*-2*-2 = -8; pow(-8, 1/3) should return -2 Test script: --------------- var_dump(pow(-8, 1/3)); Expected result: ---------------- float(-2) Actual result: -------------- float(NAN) ------------------------------------------------------------------------ -- Edit this bug report at https://bugs.php.net/bug.php?id=63136&edit=1