A population, $P$, begins with $860$ members and doubles every $13$ years. Give the constants $a$, $b$, and $T$ so that the population is represented by a function of the form $P = ab^{t/T}$, where $t$ is the time in years since the population was first measured.

$a =$ help (numbers)
$b =$ help (numbers)
$T =$ help (numbers)