Citizenship for adult child of Portuguese citizen

Hello,

I was hoping I might be able to get some guidance from an immigration expert in Portugal.

My father is Portuguese and I am interested in obtaining citizenship through him. As I understand this is pretty straightforward even as an adult, but the issue is that my father is not listed on my birth certificate because he wasn't in the country at the time of my birth.

Is there an alternate way of proving paternity? I know in the US they use DNA tests and in other countries it's as simple as getting him to swear an affidavit.

What would be the route forward in order to prove eligibility for Portugal?

Thanks in advance!

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