Step 1: Proving the base case.
Step 2: Proving that if holds for k holds for k+1
or something. Otherwise, take the marks on the chin, because there's nothing wrong with a bit of notational abuse! What you have written isn't ambiguous.
Alternatively, ask the person doing your marking what you should write. They'll know what they want...
(Was this an exam, or homework? I dock marks for silly things with my first-years, for example writing for your answer will get a big red cross through it - it should be , or . But in a test I wouldn't care (big cross through it, yes, but they'd get the marks!)).