1, 6, 20, 51, 189, 517, 2197, 4823, 14496, what comes next?
\(\small{ \begin{array}{rcr} a_1 &=& 1\\ a_2 &=& 6\\ a_3 &=& 20\\ a_4 &=& 51\\ a_5 &=& 189\\ a_6 &=& 517\\ a_7 &=& 2197\\ a_8 &=& 4823\\ a_9 &=& 14496\\ \end{array} }\)
\(\begin{array}{|cr|r|r|r|r|} \hline &1&20&189&2197&14496 \\ +&6&+51&+517&+4823&+\color{red}55555 \\ \hline =&7&7\underbrace{1}_{a_1}&70\underbrace{6}_{a_2}&70\underbrace{20}_{a_3}&700\underbrace{51}_{a_4} \\ \hline \end{array}\)
next comes \(\color{red}55555\)
