Today we will look at the form भैक्षम् nAs from श्रीमद्वाल्मीकि-रामायणम् 2.43.4

अथास्मिन् नगरे रामश्चरन् भैक्षं गृहे वसेत् । कामकारो वरं दातुमपि दासं ममात्मजम् ∥ २-४३-४ ∥

Gita Press translation – Even if Rāma were allowed to remain at his home in Ayodhyā living on alms, it would be preferable (to me in comparison to his exile). (Nay), it would be better even to consign my son to her as a slave (4).

भिक्षाणां समूहः = भैक्षम् – alms (a collection of items got from begging)
In the verses the विवक्षा is द्वितीया-एकवचनम्। Hence the form is भैक्षम्।

(1) भिक्षा आम् + अण् । By 4-2-38 भिक्षादिभ्योऽण् – To denote a collection/group, the तद्धित: affix ‘अण्’ may be optionally applied following a syntactically related पदम् in which the सन्धिः operations have been performed, provided the पदम् ends in a genitive case affix and has ‘भिक्षा’ etc (listed in the भिक्षादि-गण:) as its base.

Note: First the सूत्रम् 4-2-37 तस्य समूहः prescribes the default affix ‘अण्’। But since ‘भिक्षा’ denotes an inanimate object, the default affix ‘अण्’ is over-ruled by the affix ‘ठक्’ prescribed by the सूत्रम् 4-2-47 अचित्तहस्तिधेनोष्ठक्। Finally the सूत्रम् 4-2-38 भिक्षादिभ्योऽण् re-prescribes the affix ‘अण्’।

Note: The form ‘भैक्ष’ may also be derived स्वार्थे (without a change in meaning) from the प्रातिपदिकम् ‘भिक्षा’ by applying the affix ‘अण्’ as per the सूत्रम् 5-4-38 प्रज्ञादिभ्यश्च। Even though ‘भिक्षा’ is not listed in the प्रज्ञादि-गणः, the सूत्रम् 5-4-38 may still be applied here, since the प्रज्ञादि-गणः is considered to be a आकृति-गणः।

(2) भिक्षा आम् + अ । अनुबन्ध-लोपः by 1-3-3 हलन्त्यम्, 1-3-9 तस्य लोपः ।

Note: ‘भिक्षा आम् + अ’ gets the designation प्रातिपदिकम्‌ by 1-2-46 कृत्तद्धितसमासाश्च। This allows 2-4-71 सुपो धातुप्रातिपदिकयोः to apply in the next step.

(3) भिक्षा + अ । By 2-4-71 सुपो धातुप्रातिपदिकयोः – A सुँप् affix takes a लुक् elision when it is a part of a धातुः or a प्रातिपदिकम्।

Note: The affix ‘अण्’ is a णित् (has the letter ‘ण्’ as इत्). This allows 7-2-117 तद्धितेष्वचामादेः to apply in the next step to perform the वृद्धिः substitution in place of the letter ’इ’, which is the first vowel of the अङ्गम् ‘भिक्षा’।

(4) भैक्षा + अ । By 7-2-117 तद्धितेष्वचामादेः – The first vowel among the vowels of a अङ्गम् which is followed by a तद्धित: affix (ref. 4-1-76 तद्धिताः) which is either a ञित् (has the letter ‘ञ्’ as a इत्) or a णित् (has the letter ‘ण्’ as a इत्) takes the वृद्धि: (ref. 1-1-1 वृद्धिरादैच्) substitute.

Note: The अङ्गम् ‘भैक्षा’ gets the भ-सञ्ज्ञा by 1-4-18 यचि भम् which allows 6-4-148 यस्येति च to apply in the next step.

(5) भैक्ष् + अ । By 6-4-148 यस्येति च – When the letter ‘ई’ or a ‘तद्धित’ affix follows, the (ending) इ-वर्ण: (letter ‘इ’ or ‘ई’) or the (ending) अवर्ण: (letter ‘अ’ or ‘आ’) of the अङ्गम् with the भ-सञ्ज्ञा takes elision (लोपः)।

= भैक्ष । Note: Words ending in तद्धिताः affixes denoting a collection are used in the language in the neuter gender. Hence the प्रातिपदिकम् ‘भैक्ष’ declines like वन-शब्दः।