I thought I would refresh this with an updated version of your excellent formula that uses the new Let function

=LET(
x, SEQUENCE(N13),
y, (0=MOD(N13,x)),
z, MMULT(--(0<MOD(x*y, TRANSPOSE(x))),x^0),
p, (N13-2=z),
q, p*x*y,

N13 * PRODUCT(IF(q = 0, 1, 1 - 1 / (q) ) )

)