RINGBOM, S. Mystik und gegenstandslose Malerei. Scripta Instituti Donneriani Aboensis, [S. l.], v. 5, p. 168–188, 1970. DOI: 10.30674/scripta.67625. Disponível em: https://journal.fi/scripta/article/view/67625. Acesso em: 23 may. 2022.