**Note: This talk begins with a pre-seminar (aimed at graduate students) at 3:30–4:00. The main talk starts at 4:10.**

**Join Zoom Meeting: https://washington.zoom.us/j/95020884635Meeting ID: 950 2088 4635**

An ideal arrangement is a hyperplane arrangement defined by a lower

ideal in the poset of positive roots of a root system. This arrangement

is known to have a close connection to a regular nilpotent Hessenberg

variety. In this talk, I will show that we can compute the cohomology

ring of a regular nilpotent Hessenberg variety from the logarithmic

derivation module of ideal arrangement. It time allows, I will also

talks about monomial basis of these cohomology rings in type A. The talk

is based on a joint work with T. Abe, T. Horiguchi, M. Masuda, T. Sato

and a joint work with M. Harada, Horiguchi, M. Precup and J. Tymoczko.