(k+1)th, (k+1)st, k-th+1, or k+1?

Solution 1:

From the Handbook of Writing for the Mathematical Sciences section 5.5 p. 63:

Here are examples of how to describe the position of a term in a sequence relative to a variable k:

kth, (k+1)st, (k+2)nd, (k+3)rd, (k+4)th, … (zeroth, first, second, third, fourth, …)

Generally, to describe the term in position k±i for a constant i, you append to (k±i) the ending of the ordinal number for position i (th, st, or nd), which can be found in a dictionary or book of grammar."

So the formal answer is that it should be:

(k+1)st

Solution 2:

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