U+212C Script Capital B

The Wikipedia has the following information about this codepoint:

In the mathematical field of complex analysis, the Bloch space, named after French mathematician André Bloch and denoted B {displaystyle {mathcal {B}}} or ℬ, is the space of holomorphic functions f defined on the open unit disc D in the complex plane, such that the function

( 1 − | z | 2 ) | f ′ ( z ) | {displaystyle (1-|z|^{2})|f^{prime }(z)|}

is bounded. B {displaystyle {mathcal {B}}} is a Banach space, with the norm defined by

‖ f ‖ B = | f ( 0 ) | + sup z ∈ D ( 1 − | z | 2 ) | f ′ ( z ) | . {displaystyle |f|_{mathcal {B}}=|f(0)|+sup _{zin mathbf {D} }(1-|z|^{2})|f'(z)|.}

This is referred to as the Bloch norm and the elements of the Bloch space are called Bloch functions.