Can $\,9\!\cdot\!10^n+4\,$ be a perfect square? I think $\,9\!\cdot\!10^n+4\,$ can be a perfect square, since it is $0 \pmod 4$ (a quadratic residue modulo $4$), and $1 \pmod 3$ (also a quadratic residue modulo  $3$).
But when I tried to find if $\;9\!\cdot\!10^n+4\,$ is a perfect square, I didn’t succeed. Can someone help me see if $\;9\!\cdot\!10^n+4\,$ can be a perfect square ?
 A: Note that if $$9\!\cdot\!10^n+4=m^2\implies (m+2)(m-2)=9\!\cdot\!10^n$$
Note that $5^n$ must divide either $m+2$ or $m-2$.
If that happens, the rest of the factors are not big enough to maintain the difference of $4$ as $\left|5^n-9\!\cdot\!2^n\right|>4$ for $n\geqslant3$.
A: If you reduce mod $11$ you get $(-2)(-1)^n+4 \equiv 2$ or $6 \pmod{11}$.  Neither $2$ nor $6$ is a quadratic residue mod $11$.
A: Assume that $9\cdot10^n+4\equiv4$ is a perfect square.
$9\cdot10^n+4\equiv4\pmod9$, so $9\cdot10^n+4$ can be represented as $(9m-2)^2=81m^2-18m+4$ or $(9m+2)^2=81m^2+18m+4$, where $m\in\mathbb{N}$.
If $9\cdot10^n+4=81m^2-18m+4$, $$10^n=9m^2-2m=m(9m-2)$$
This means that $m$ and $9m-2$ must be powers of $10$.
Clearly, $9m-2>m$ because $m>0$. If $m≠1$, $m\equiv0\pmod{10}$,  but then $9m-2\equiv8\pmod{10}$, which doesn't work. If $m=1$, $9m-2=7$, which also doesn't work.
If $9\cdot10^n+4=81m^2+18m+4$, $$10^n=9m^2+2m=m(9m-2)$$
This means that $m$ and $9m+2$ must be powers of $10$.
Clearly, $9m+2>m$ because $m>0$. If $m≠1$, $m\equiv0\pmod{10}$,  but then $9m+2\equiv2\pmod{10}$, which doesn't work. If $m=1$, $9m+2=11$, which also doesn't work.
Therefore, there is a contradiction and so $9\cdot10^n+4$ cannot be a perfect square.
A: Comment:
Lets try construction such a number. Suppose we have:
$k^2-9\times 10^n=4$
We use following known Pell's equation:
$x^2-Dy^1=1$
For $D=10$ we have $x=19$ and $y=6$ such that:
$19^2-10\times 6^2=1$
multiplying both sides by $2^2$ we get"
$38^2-10\times 12^2=4$
we rewrite this as:
$38^2-9\times(4^2\times 10)=4$
multiplying both sides by $25^2$ we get:
$(25\times 38)^2-9\times 10^5=4\times 25^2=2500=2496+4$
Or:
$[(25\times 38)^2-2496]-9\times 10^5=4$
Or generally:
$A=[(25\times 38)\times 10^m-(25\times 10^{2m}+96)]$
here in equation $k^2-9\times 10^n=4$, $n=2m+1$
$A$ must be perfect square.May be by brute force we can find such a number.
