math.h does define: -

http://www.nongnu.org/avr-libc/user-manual/group__avr__math.html -

#define M_PI 3.14159265358979323846 /* pi */

#define M_PI_2 1.57079632679489661923 /* pi/2 */

#define M_PI_4 0.78539816339744830962 /* pi/4 */

#define M_1_PI 0.31830988618379067154 /* 1/pi */

#define M_2_PI 0.63661977236758134308 /* 2/pi */

#define M_2_SQRTPI 1.12837916709551257390 /* 2/sqrt(pi) */

unfortunately math.h does not define 180/PI and PI/180 for radians <> degrees conversion

(windows calc)

#define M_180_PI 57,295779513082320876798154814105 /*180/pi */

#define M_PI_180 0,01745329251994329576923690768489 /* pi/180 */

// the arduino does only support about 7 digits of float precision so the above numbers are overkill (unless the arbitrary precision lib is used )