The affix a comes after 1. ṛk 2. pur 3. ap 4. dhur and 5. pathin when finals is compound, but not so when dhur is in conjunction with akṣa,
The anuvritti of the word Bahuvrihi does not enter here. It applies to all compounds in general. The word अनक्षे qualifies धुर्, as it cannot qualify the words ऋक् &c.
Thus अनृचः बह्वृचः,अर्द्धर्चः, pur: as, ललाटपुरम्, नान्दीपुरम् ॥ ap as, द्वीपम् = (द्विर्गता आपो यस्मिन्) = द्वि + अप् = द्वि + ईप् 6.3.97 = द्वीपं, अन्तरीपम्, समीपम् ॥ dhur as, राजधुरा, महाधुरा ॥ pathin, as, जलपथः ॥
Why do we say \when not in conjunction with अक्ष?\ Observe, अक्षस्य धूः = अक्षधूः, दृढधूरक्षः ॥
The words अनृचः and बह्वृचः apply to students of Rik: thus अनृचः means \a boy who has not read the Rik\. बह्वृचः \a Brahmana who has read many Rig verses\. The affix अ is not applied when it refers to the Vedas, as अनृक साम, बह्वृक् सूक्तम् ॥,