declining numerals in Old English

Question

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I read in the Anglo-Saxon Chronicle, in the famous extract about Cynewulf and Cyneheard (year 755) :

[...] Ond se Cynewulf oft miċlum ġefeohtum feaht uuiþ Bretwālum, ond ymb .xxxi. wintra þæs þe hē rīċe hæfde hē wolde ādrǣfan ānne æþeling [...]

(my translation) And Cynewulf fought great fights with the British and after 31 winters within which he had the kingdom he wanted to exile a nobleman [...]

My question : how was "ymb .xxxi. wintra" expressed ?

".xxxi. wintra" being a masculine genitive plural (ymb + genitive), what's the masculine genitive plural of "ān and þrītiġ" (=31) ?

My guess : ānum and þrītiġ

• In the Guide To Old English (Mitchell & Robinson; sixth edition) I read (§ 83) that "declined strong, ān means 'one'; when declined weak āna, it usually means 'alone'. So, I guess that ān will follow the strong declension. But what's the genitive plural of such a form ? Is it ānum, as expected if I follow the usual declension of the strong adjectives ?
• In the Guide To Old English (Mitchell & Robinson; sixth edition) I read (§ 194) that "the cardinal numbers can be used as adjectives agreeing with a noun, e.g. [...] mid XXXgum cyningum 'with thirty kings'." Since ".xxxi." isn't followed by any ending, I assume that þrītiġ is the expected word (not þrītigum). Is it correct ?

Answer 1

Quote from An Introduction to Old English by George Leslie Brook, page 51:

§132 The cardinal numerals from 4 to 19 generally remain uninflected when they stand before a noun; when they follow a noun and when they are used as nouns they are declined like the plurals of nouns of the i-declension; nom. and acc. m. and f. -e, n. -u; gen. -a, dat. -um.

§133 The ending -tig of the numerals from 20 to 120 was originally a noun; the Old English numerals in -tig could be used either as nouns or as adjectives. When they are used as nouns the genitive ends in -es; when used as adjectives they are either uninflected or declined like manig. Since they were originally nouns, numerals in -tig, like hund and þūsend, may be followed by a noun in the partitive genitive. Examples are fiftiges elna lang, fifty ells long; twentig scēapa, twenty sheep. Hund, hundred, is generally uninflected, but it occasionally has dative forms in -e, -um. þūsend, thousand, is sometimes uninflected but is more often declined as a neuter noun.

According to the above quoted rules, the numeral in ymb .xxxi. wintra would most often remain uninflected, but it could also be declined like manig, many, that is like the strong adjectives. As ān, one is part of the number thirty-one it also would not be declined separately. So, I think the correct and most usual pronunciation of ymb .xxxi. wintra would be ymb an and þrittig wintra.