Doubt about ther POWPOW2

#1

ll a1 = (n == 0) ? 1 : 0;

in video says it is precomputed factorial .if n == 0 then 1 understood but if 4 or 6! then why else condition is zero

and

do_func();
do_func2();

how these function is actually working.I can’t understood?
calc_fact(); == it is understood that for the factorial no problem in video the other two problem is not mentioned why is it here.

#2

a2 = (a2 * c_pow(41, fact_41_p[2 * n] - 2 * fact_41_p[n], 1681)) % 1681;

#3

Significance of this line a2 = (a2 * c_pow(41, fact_41_p[2 * n] - 2 * fact_41_p[n], 1681)) % 1681;

#4

@kaushikghosh199832
Can you elaborate? I am unable to understand your concern