Question 39
a sequence $(a_n)_{n=1}^{\infty}$ of natural numbers has the property that for any $n \ge 1$, $a_{n+1} = a_n + b_n$, where $b_n$ is the number having the same digits as $a_n$, but in the reverse order (unlike $a_n$, the number $b_n$ may start with one or more zeroes in the decimal notation). For instance, if $a_1 = 170$ we have $a_2 = 170 + 71 = 241$, $a_3 = 241 + 142 = 383$, and so on.
a. Can the number $a_6$ be prime?
b. Can the number $a_7$ be prime?
a. Can the number $a_6$ be prime?
b. Can the number $a_7$ be prime?