## Prime index of a palindromic hexagonal number

A hexagonal number is a polygonal number of the form   $n \, (2 \, n - 1)$

$9561677372927686361$   is a 19-digital prime index of the 39-digital palindromic hexagonal number:

$9561677372927686361 \,(2 \times 9561677372927686361 - 1)$
$= \; 182851348367914603505306419763843158281$