View Compilation Error

0_0_8918146_21520.cpp: In function `int fun(double*, int, int, double)':
0_0_8918146_21520.cpp:23: error: `fabs' undeclared (first use this function)
0_0_8918146_21520.cpp:23: error: (Each undeclared identifier is reported only once for each function it appears in.)
0_0_8918146_21520.cpp: In function `int main()':
0_0_8918146_21520.cpp:109: error: `fabs' undeclared (first use this function)