Am 11.05.2020 um 23:38 schrieb Dominik Wujastyk via INDOLOGY <indology@list.indology.info>:

I always thought the first, with ś, was correct, and the second was just an orthographic error. Then I saw "-koṣa" on the title page of Ingalls's Subhāṣitaratnakoṣa. He must have thought about this.

So which is "right" and why?

In his Indian Lexicography Claus Vogel points to Amarakośa 3.3.221 (sometimes 3.3.222 in some editions) and the commentaries that seems to justify the spelling koṣa. See Vogel: Indian Lexicography (1st edition), p. 303, note 1 (at this moment I don’t have access to the 2nd edition).

Hope it helps,

Peter Wyzlic

Institut für Orient- und Asienwissenschaften

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