Today we will look at the forms वैश्रवणम् mAs and रावण mVs from श्रीमद्वाल्मीकि-रामायणम् 3.48.21 and 3.48.22.

एवमुक्ता तु वैदेही क्रुद्धा संरक्तलोचना । अब्रवीत्परुषं वाक्यं रहिते राक्षसाधिपम् ।। ३-४८-२० ।।
कथं वैश्रवणं देवं सर्वभूतनमस्कृतम् । भ्रातरं व्यपदिश्य त्वमशुभं कर्तुमिच्छसि ।। ३-४८-२१ ।।
अवश्यं विनशिष्यन्ति सर्वे रावण राक्षसाः । येषां त्वं कर्कशो राजा दुर्बुद्धिरजितेन्द्रियः ।। ३-४८-२२ ।।

Gita Press translation “Enraged when spoken to as aforesaid, Sītā (a princess of the Videha territory) for her part with blood-red eyes addressed the following harsh words to Rāvaṇa (the suzerain lord of ogres) in that lonely place : – (20) “How after calling god Kubera (son of Viśravā), who is hailed by all gods, your (half-) brother, do you seek to perpetrate a foul deed ? (21) All ogres, O Rāvaṇa, will inevitably perish inasmuch as you – who are (so) hardhearted and evil-minded and have not been able to subdue your senses – are their ruler.(22)”

The above verses have previously appeared in the following post – विनशिष्यन्ति-3ap-लृँट्

विश्रवसोऽपत्यम् (पुमान्) = वैश्रवणः, रावणः – a (male) descendant of Viśravā. Note: वैश्रवणः can refer to either Kubera or Rāvaṇa, since both were sons of Viśravā. Here in the verses वैश्रवणः refers to Kubera.
In the verses, the विवक्षा for वैश्रवणम् is द्वितीया-एकवचनम्। And the विवक्षा for रावण is सम्बुद्धिः।

(1) विश्रवण/रवण ङस् + अण् । By 4-1-112 शिवादिभ्योऽण् – To denote the sense of अपत्यम् (descendant) the तद्धित: affix ‘अण्’ may be applied optionally following a syntactically related पदम् in which the सन्धिः operations have been performed, provided the पदम् ends in a sixth case affix and has ‘शिव’ etc (listed in the शिवादि-गण:) as its base.
Note: ‘विश्रवण’ and ‘रवण’ are listed in the शिवादि-गणः, implying that ‘विश्रवस्’ takes the substitution ‘विश्रवण’/’रवण’ when followed by the affix ‘अण्’ in the sense of अपत्यम् (descendant).
Note: First the सूत्रम् 4-1-83 प्राग्दीव्यतोऽण् prescribes the default affix ‘अण्’ which is over-ruled by the affix ‘इञ्’ (prescribed by the सूत्रम् 4-1-95 अत इञ्) and finally the सूत्रम् 4-1-112 शिवादिभ्योऽण् re-prescribes the affix ‘अण्’ because ‘विश्रवण’ and ‘रवण’ are specifically listed in the शिवादि-गण:।
Note: The अनुवृत्तिः of ‘गोत्रे’ from the सूत्रम् 4-1-98 गोत्रे कुञ्जादिभ्यश्च्फञ् does not come into this सूत्रम् 4-1-112. It stops at the prior सूत्रम् 4-1-111 भर्गात्‌ त्रैगर्ते।
As per the सूत्रम् 4-1-92 तस्यापत्यम् – Following a syntactically related पदम् in which the सन्धिः operations have been performed and which ends in a sixth case affix the तद्धिता: affixes already prescribed (by the prior rules 4-1-83 प्राग्दीव्यतोऽण् etc), as well as those that are going to be prescribed (by the following rules 4-1-95 अत इञ् etc), may be optionally applied to denote the sense of अपत्यम् (descendant.)

(2) विश्रवण/रवण ङस् + अ । अनुबन्ध-लोपः by 1-3-3 हलन्त्यम्, 1-3-9 तस्य लोपः।

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

(3) विश्रवण/रवण + 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.

(4) वैश्रवण/रावण + 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 ‘वैश्रवण/रावण’ gets the भ-सञ्ज्ञा by 1-4-18 यचि भम् which allows 6-4-148 यस्येति च to apply in the next step.

(5) वैश्रवण्/रावण् + 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)।

= वैश्रवण/रावण । The प्रातिपदिकम् ‘वैश्रवण’ as well as ‘रावण’ declines like राम-शब्दः।

The विवक्षा for वैश्रवणम् is द्वितीया-एकवचनम्।
(6a) वैश्रवण + अम् । By 4-1-2 स्वौजसमौट्छष्टाभ्याम्भिस्ङेभ्याम्भ्यस्ङसिभ्याम्भ्यस्ङसोसाम्ङ्योस्सुप्‌।
Note: 1-3-4 न विभक्तौ तुस्माः prevents the ending letter ‘म्’ of the affix ‘अम्’ from getting इत्-सञ्ज्ञा।

(7a) वैश्रवणम् । By 6-1-107 अमि पूर्वः – In place of a preceding अक् letter and the following vowel (अच्) belonging to the affix ‘अम्’ there is a single substitute of that preceding अक् letter.

And the विवक्षा for रावण is सम्बुद्धिः।
Note: By 2-3-49 एकवचनं सम्बुद्धि: – The nominative singular affix (‘सुँ’) when used in a vocative form gets the designation सम्बुद्धि:।

(6b) (हे) रावण + सुँ । By 4-1-2 स्वौजसमौट्छष्टाभ्याम्भिस्ङेभ्याम्भ्यस्ङसिभ्याम्भ्यस्ङसोसाम्ङ्योस्सुप्‌। By 2-3-47 सम्बोधने च – A first case affix (‘सुँ’, ‘औ’, ‘जस्’) is used to denote ‘address’ (in addition to the meaning of the nominal stem) also. Note: सम् (सम्मुखीकृत्य) बोधनम् (ज्ञापनम्) = सम्बोधनम्। सम्बोधनम् means drawing someone’s attention (to inform him/her of something.)

(7b) (हे) रावण + स् । अनुबन्ध-लोप: by 1-3-2 उपदेशेऽजनुनासिक इत् and 1-3-9 तस्य लोपः।

(8b) (हे) रावण । By 6-1-69 एङ्ह्रस्वात्‌ सम्बुद्धेः – Following a अङ्गम् ending in ‘एङ्’ (letter ‘ए’ or ‘ओ’) or a short vowel, a consonant is dropped if it belongs to a सम्बुद्धि: affix.

We can similarly derive the following –
१) ककुत्स्थस्यापत्यम् (पुमान्) = काकुत्स्थः (रामः) – a (male) descendant of (the king) Kakutstha. It refers to Śrī Rāma.