Apastamba ( 5th c. BC), was an Indian priest/scholar mathematician associated with the black Yajur Veda (the others being Baudhayana, Vaikhanasa, Satyasadha, Bharadhvaja and Agnivesa).
His Kalpasutra is an important part of the Hindu canon. It is divided into thirty chapters. The first 24 are about the performance of the Srauta sacrifices (Srautasutra). The 26th chapter is on the Grihya rites (Grihyasutra). The thirtieth is about geometry of altar construction (Sulbasutra). One of the most notable facts found in this Sulbasutra is an approximation of the square root of 2. It must be noted that like all the other Sulbasutras, it is not known how much of the mathematical development therein is original and how much is merely a transcription of existing knowledge.