What is the pseudonym used by the author in The Dark Half?

What is the pseudonym used by the author in The Dark Half? George Stark is the answer.

Thad Beaumont is the author at the center of Stephen King's 1989 novel. His serious literary books achieve limited success, while the violent crime novels he writes under the name George Stark become bestsellers. Stark's books feature the psychopathic killer Alexis Machine and represent the darker, more commercially successful side of Thad's writing.

When a college student exposes Thad as George Stark, Thad and his wife stage a mock burial for the pseudonym, complete with a grave and an epitaph. The publicity is meant to retire Stark, but the character mysteriously takes physical form and begins murdering people connected with the revelation. Sheriff Alan Pangborn investigates while evidence points toward Thad.

George Stark is not the same as Richard Bachman, the real-world pseudonym King used for several novels. King did, however, draw inspiration for the name Stark from Richard Stark, a pen name of writer Donald E. Westlake. That overlap explains why Richard Bachman is a tempting but incorrect option.