Exp coeff properties 20250325

[IshiguroYoshihiro]

Motivation for this change

Some properties of exp_coeff.

Checklist

• [x] added corresponding entries in CHANGELOG_UNRELEASED.md

• [ ] added corresponding documentation in the headers

Reference: How to document

Merge policy

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• PRs with several commits that make sense individually and that all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

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