Let E be an
(L1,L∞)-interpolation space. Then (TE(t)f)(x) = f(e−tx) defines a group on E. It is
strongly continuous if and only if E has order continuous norm. In any case, a
generator AE can be associated with TE. It is shown that its spectrum is the strip
{αE≤Reλ ≤αE}, where αE and αE are the Boyd indices of E. The operator
BE= (AE)2 generates a holomorphic semigroup which governs the Black–Scholes
partial differential equation ut= x2uxx+ xux, whose well-posedness, spectrum and
asymptotics in E are studied.