G1.1 Find the unit digit of $\left(2^{13}+1\right)\left(2^{14}+1\right)\left(2^{15}+1\right)\left(2^{16}+1\right)$.
$$
\begin{array}{l}
2^{10}=1024,2^{11}=2048,2^{12}=4096,2^{13}=8192,2^{14}=16384,2^{15}=32768,2^{16}=65536 \\
\left(2^{13}+1\right)\left(2^{14}+1\right)\left(2^{15}+1\right)\left(2^{16}+1\right)=8193 \times 16385 \times 32769 \times 65537 \equiv 3 \times 5 \times 9 \times 7 \equiv 5(\bmod 10)
\end{array}
$$

Unit digit $=5$