According to https://unix.superglobalmegacorp.com/ https://unix.superglobalmegacorp.com/ it was in 3bsd libm -https://unix.superglobalmegacorp.com/cgi-bin/cvswe

According to https://unix.superglobalmegacorp.com/ https://unix.superglobalmegacorp.com/ it was in 3bsd libm -https://unix.superglobalmegacorp.com/cgi-bin/cvsweb.cgi/3BSD/lib/libm/atan.c?rev=1.1.1.1;content-type=text%2Fplain;only_with_tag=MAIN https://unix.superglobalmegacorp.com/cgi-bin/cvsweb.cgi/3BSD... 3BSD was released at the end of 1979 according to wikipedia, and that's just the oldest BSD source I could find, so it may have been in even older versions. Before ANSI/ISO C I don't think there was really a "math.h" to look for - as it may be different on different systems, but as so many things derive from BSD I wouldn't be surprised if it was "de-facto" standardised to what that supported.