Rational $\theta$-parallelogram envelopes via $\theta$-congruent elliptic curves

We introduce a new generalization of $\theta$-congruent numbers by defining the notion of rational $\theta$-parallelogram envelope for a positive integer $n$, where $\theta \in (0,\pi )$ is an angle with rational cosine. Then, we study more closely some problems related to the rational $\theta$-parallelogram envelopes, using the arithmetic of algebraic curves. Our results generalize the recent work of T.~Ochiai, where only the case $\theta =\pi \mathrm{/}2$ was considered. Moreover, we answer the open questions in his paper and their generalizations for any Pythagorean angle.