$\backslash$end$\{$array$\}]$

$a_n=Hn+1$

$=H(n-2+1)+1$

$=Hn-2+H+1$

$=H^2(n-3+1)+H+1$

$=H^2-3+H^2+H+1$

$=H^3(n-4+H)+H^2+H+1$