Function with complicated argument constraint with default value

Question

I need to define a function, say f with an argument which is integer, positive and with a default value say 2. I tried:

f[n_Integer?Positive:2]:=n^4-3*n+5

or

f[n_Integer:2?Positive]:=n^4-3*n+5

but it didn't work.

Answer 1

In this case, the first : which is the shorthand of Pattern cannot be omitted (yeah n_ is actually a shorthand of n : _):

f[n : _Integer?Positive : 2] := n^4 - 3 n + 5