Denote the elements of the field $F_{4}$
by $\{0, 1, w, w + 1\}$
, where $w$
satisfies the following equation with coefficients in $F_{2}: w^{2} + w + 1 = 0$
.
Mothman is a recurrent double sequence defined by $a(i, 0) = a(0, j) = 1$
and
$a(i, j) = f(a(i, j-1), a(i-1, j-1), a(i-1, j))$
,
where $f(x, y, z) = x^{2} + (w + 1) y^{2} + z^{2}$
.
This recurrent double sequence can be also obtained using a system of substitutions of type 2 -> 4 with 15 rules, as it follows.