Is an induction inside of an induction allowed in a proof?

Note: I'll be putting letters before each statement I discuss in order to easily reference them.

If I'm proving a statement with induction, can I use induction on a statement that I derive inside said proof? For example:

Prove for all positive integers n >= 3, that:

A: n! > ((n-1)! + (n-2)! + (n-3)!)

To solve this, I checked that the inequality holds for the base case n = 3, which it does. Then in the inductive step, I assumed the inequality was correct.

Before trying to prove the inequality for n+1, I algebraically simplified the assumption (A) into:

B: n^3 - 4n^2 + 4n - 1 > 0

And then my goal is to prove:

C: (n+1)^3 - 4(n+1)^2 + 4(n+1) - 1 > 0

Using assumption B, I simplified C into:

D: 3n^2 - 5n + 1 > 0

Now if I solve D, my entire proof is correct (i.e., I've proved A).

The problem is, I don't know how to prove D without using induction. So I used induction and was able to successfully prove D for all n >= 3.

My question is was I allowed to use a separate induction in the inductive step of the original proof?

• You don't need induction but may use it. – ancientmathematician Apr 25 '18 at 9:19
• Try completing the square to find the minimum point on that curve and then you can show it is always positive the values of $n$ you’re interested in – PhysicsMathsLove Apr 25 '18 at 9:25

You need not require induction to prove that $3n^2-5n+1$ > $0$. Remember, it is given that $n>=3$. Therefore , $3n^2 -5n$ will always be a positive quantity and adding $1$ will always yield a quantity greater than $0$.
Hence, $3n^2-5n+1$ will always be greater than 0.