# Thread: Show 2018th term of Sylvester's Sequence isn't a perfect square

1. ## Show 2018th term of Sylvester's Sequence isn't a perfect square

A sequence $\displaystyle T_1, T_2, T_3 ...$ is defined by:
$\displaystyle T_1 = 1$
$\displaystyle T_2 = 2$
$\displaystyle T_{n+1} = 1 + \prod_{i=1}^{n} T_{i}$ for all integers $\displaystyle n \geq 2$

Prove that $\displaystyle T_{2018}$ is not a perfect square.

For context, this was given to me by a student, and is the last part of a four part question. The previous parts were:

(a) What is the value of $\displaystyle T_5$? (Ans: 1 + 1*2*3*7 = 43)
(b) Prove that $\displaystyle T_{n+1} = T_n^2 - T_n + 1$ for $\displaystyle n \geq 2$
(Ans:
Observe that $\displaystyle T_n^2 - T_n = T_n(T_n - 1) = T_n($$\displaystyle \prod_{i=1}^{n-1} T_{i}) = T_{n+1} - 1$
then add one on both sides)

(c) Prove that $\displaystyle T_n + T_{n+1}$ is a factor of $\displaystyle T_nT_{n+1} - 1$ for all integers $\displaystyle n \geq 2$
(Ans:
$\displaystyle T_nT_{n+1} - 1 = T_n^3 - T_n^2 + T_n - 1 = (T_n^2 + 1)(T_n - 1) = (T_n + T_{n+1})(T_n - 1)$

On part (d), which is to prove that $\displaystyle T_{2018}$ is not a perfect square, I'm stumped. How might one approach this? I might be missing something obvious as the structure of the question leads me to believe (c) is a fact I can use.
Either I can show the square root of a representation of $\displaystyle T_{2018}$ isn't rational, or show $\displaystyle T_{2018}$ is between two consecutive perfect squares?

2. ## Re: Show 2018th term of Sylvester's Sequence isn't a perfect square

sequence of unit digits of the $T_i$

$\displaystyle \{1,2,3,7,3,7,3,7,3,7,3,7,3,7,\text{...}\}$

so it looks like $T_{2018}$ ends in a $7$