Yīxíng 一行 (683–727) — Tang-dynasty polymath monk, mathematician-astronomer of the imperial court, and the principal Chinese disciple of Śubhakarasiṃha 善無畏 善無畏 and Vajrabodhi 金剛智 金剛智. Lay name Zhāng Suì 張遂; native of Jùlù 鉅鹿 (modern Hebei). Grandson of the early-Tang general Zhāng Gōngjǐn 張公謹 and son of Zhāng Lǐn 張懍. Posthumous title Dàhuì Chánshī 大慧禪師 (“Chán Master of Great Wisdom”), conferred by Emperor Xuánzōng.
He took ordination at Sōngshān 嵩山 under the Northern Chán patriarch Pǔjí 普寂 (a successor of Shénxiù 神秀); subsequently studied Vinaya, then journeyed to Tiāntáishān 天台山 to study astronomy and calendrical mathematics under the recluse Yìnzhēn 印真. Recalled to Chángān by Emperor Xuánzōng in 717 to lead the imperial calendrical reform, he produced the Dàyǎnlì 大衍曆 calendar of 728 (presented posthumously) — the standard Tang astronomical calendar that incorporated his astronomical-instrument innovations and replaced the Línde 麟德 calendar.
In Esoteric Buddhism his unique role is as the bridge between the two great Tang transmissions: he received the Garbhadhātu initiation from Śubhakarasiṃha (whose oral teachings on the Mahāvairocana-sūtra he transcribed as the Dàrì-jīng shū 大日經疏 KR6j0080, T39n1796 — the foundational Esoteric commentary in East Asia) and the Vajradhātu transmission from Vajrabodhi. He thus became the first Chinese master of the dual-mandala 兩部 transmission that defines the East Asian Esoteric tradition.
His role as Śubhakarasiṃha’s amanuensis on the Mahāvairocana-sūtra translation (KR6j0001, T18n0848) is recorded in his colophon and confirmed by the Sòng Gāosēng-zhuàn (T50n2061). The Dàyǎn-lì survives in fragments embedded in the Xīn Tángshū 新唐書 and Jiù Tángshū 舊唐書 lìzhì 曆志 chapters.
He died on the 8th day of the 10th month of Kāiyuán 15 = 727 CE, while accompanying Emperor Xuánzōng to Xīnfēng 新豐, in his 45th year. The hagiography reports a serene śūnya-samādhi death with the formulaic signs of a saintly monk’s parinirvāṇa.
Apart from his Esoteric works, his astronomical writings include the Dàyǎn lìshù lìchéng 大衍曆術立成, Tiānwén yāojié 天文要訣, Zhāngshuō suǒshuō 漲說所說, and several lost works on calendrical astronomy listed in the Tángshū yìwénzhì 唐書藝文志.
Source: DILA Buddhist Person Authority A000008; Sòng Gāosēngzhuàn 宋高僧傳 fasc. 5 (T50n2061_p0732c–p0734b); Jiù Tángshū 舊唐書 fasc. 191 Fāngjì zhuàn 方伎傳; Xīn Tángshū 新唐書 fasc. 27 Lìzhì 曆志; Chen Jinhua, Yixing’s Astronomical Works and Their Buddhist Context (École française d’Extrême-Orient, 2016).