Does every large prime satisfy $\sum_{\substack{ab\equiv1\pmod p}}\frac{1}{\sqrt{ab}}\longrightarrow 5 ? $
问题内容
My experimental observation suggests that the sum of the reciprocals of the square roots of the products of all multiplicative-inverse pairs modulo a prime $p$ tends to $5$ as $p \to \infty$.
More specifically, for each prime $p$, consider the pairs $(a,b)$ satisfying $1\le a,b\le p-1$ and $ab\equiv1\pmod p$. Then
$$ \sum_{\substack{1\le a,b\le p-1\\ ab\equiv1\pmod p}}\frac{1}{\sqrt{ab}} \longrightarrow 5. $$
Can this be proved?
Source code:
%%time
import math, random
P = {
10: 29, 100: 541, 1_000: 7919, 10_000: 104729,
100_000: 1299709, 1_000_000: 15485863,
10_000_000: 179424673, 100_000_000: 2038074743,
1_000_000_000: 22801763489
}
def S_exact(p):
inv = [0] * p
inv[1] = 1
s = 1.0
for a in range(2, p):
inv[a] = p - (p//a) * inv[p % a] % p
s += 1 / math.sqrt(a * inv[a])
return s
def S_mc(p, N=300_000):
lo, hi = math.sqrt(2), math.sqrt(p)
total = 0.0
for _ in range(N):
a = int((lo + (hi-lo)*random.random())**2)
b = pow(a, -1, p)
q = ((math.sqrt(a+1)-math.sqrt(a)) +
(math.sqrt(b+1)-math.sqrt(b))) / (2*(hi-lo))
total += 1 / (math.sqrt(a*b) * q)
return 1 + total/N
for n, p in P.items():
s = S_exact(p) if p < 2_000_000 else S_mc(p)
print(f"n={n:>10,} p={p:>14,} S(p)={s:.8f} error={s-5:+.8f}")
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