# Even more mysterious sum equaling to $\frac{7(p^2-1)}{24} - h(-p)$ where $p \equiv 3 \pmod{4}$

This is a direct follow-up to this question, which I think is interesting enough to be a separate problem. Please check that post out too in case there are ideas there that might transfer here.

In the post linked, they found numerically that for odd primes $$p \equiv 1 \pmod{4}$$, the sum of all $$n$$ such that $$(n^2+2n+1 \ \mathrm{mod} p) < (n^2 \ \mathrm{mod} p)$$ is exactly $$\frac{7}{24}(p^2-1)$$. It is natural to generalise this for $$p \equiv 3 \pmod{4}$$, and you won't believe what I found! Here's some Sage code:

x = var('x')
for p in prime_range(6, 1001):
if p % 4 != 3: continue
ns = [n for n in range(p) if (n^2 + 2*n + 1) % p < (n^2) % p] # your condition
R.<t> = NumberField(x^2 + p) # setup Q[sqrt(-p)]
assert sum(ns) == (p^2 - 1) * 7 / 24 - R.class_number() # observed = ???


As annotated above, I conjecture that for odd primes $$p > 3$$ satisfying $$p \equiv 3 \pmod{4}$$, we have

$$\sum_{0 \leq n < p} n \cdot [(n+1)^2\,\mathrm{mod}\, p \leq n^2\,\mathrm{mod}\, p] = \frac{7}{24}(p^2-1)-h(\sqrt{-p}).$$

Feel free to play with the Sage script above. I suspect that there might be some "symmetry" or relation with quadratic residues, since it is well known that the class number is (-1 / p) times sum of n * (n | p).

I will set a bounty when I can.

• Yes, this might be finally a simple method to calculate the class number. Check this desmos: desmos.com/calculator/gnfkwahnbf you rotate and shift by 1; they line up almost perfectly only differ in three places: $0$, middle and $p$. Feb 15, 2023 at 21:16
• PARI/GP will be a much better tool for this purpose than Desmos. Feb 16, 2023 at 14:37
• What is $h(-p)$ and why do you have $h(-\sqrt{p})$ later ? Feb 16, 2023 at 14:39
• Combining the techniques from my answer in the linked problem, with Will Jagy's formula below, should yield this. Note that the only thing mysterious in the calculation with $p \equiv 3 \pmod 4$ will be the sum of quadratic residues which should exactly be taken care of by that class number formula. Feb 16, 2023 at 20:28
• Well, it works. Highlights include, with prime $q = 4 w + 3,$ $$\sum_{j=1}^w \left\lfloor \sqrt{jq} \right\rfloor = 2w^2 -\sum_{x=1}^{2w} \left\lfloor \frac{ x^2}{q} \right\rfloor$$ and $$\sum_{x=1}^{2w} \left\lfloor \frac{ x^2}{q} \right\rfloor = \frac{(q-1)(q-11)}{24} + \frac{1+ h( \sqrt {-q} )}{2}$$ Feb 20, 2023 at 4:13
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