indefinite article - Should I use "a" or "an" when I abbreviate a word?

Taking notes, I was going to write this:

therefore p′ is a shortest path.

But the topic is APSP which stands for "all pairs shortest paths" so I decided to just write SP instead and...

Is it written with a like this

therefore p′ is a SP

or with an like this (because in my head I would "read" the S)

therefore p′ is an SP

Answer

Whether you say a or an is determined by the pronunciation of the next word, and nothing else. If the next word begins with a consonant sound (not necessarily a consonant letter!), you say a, and if it begins with a vowel sound (not necessarily a vowel letter!), you say an.

So yes, you are correct: if you write or read "SP", you would say "an SP", because "ess" begins with a vowel sound.