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geansaidhjumper, sweatermasculine noun
the
an geansaidh
after a preposition
a' gheansaidh
of the
a' gheansaidh
plural
geansaidhean
the (plural)
na geansaidhean

Every form but the headword, the genitive and the plural is worked out by the spelling rules in the grammar guide, so what you see here is what the guide teaches. Checked by a fluent speaker.