BL database, 1 Nov. 2002 (ref.: Rebello, Pedro Juli ao)
Encyc. Brit., 15th ed., c1983 (John XXI; orig. name, Petrus Juliani, also called Peter of Spain; b. c. 1210-20, Lisbon; d. 5/20/1277, Viterbo, Italy; pope, 1276-77; wrote one of the most widely-used medieval textbooks on logic, Summulae logicales)
Encyc. of philosophy, c1967 (Peter of Spain, or Petrus Hispanus, Petrus Juliani (1210/1220-1277); logician, philosopher, pope; author of Summulae logicales)
His Qui in comincia illibro chiamato Thesoro de poueri, ca. 1497 t.p. (Piero Spano)
His Tractatus, called afterwards, Summule logicales, 1972, 1973 t.p. (Peter of Spain (Petrus Hispanus Portugalensis)) p. ix, etc. (identification with Pope John XXI disputed; may have been Petrus Alfonsi, O.P., or Petrus Ferrandi Hispanus (d. 1254-1259?))
John XXI, Pope. Il tesoro dei poveri, c2007 t.p. (Pietro Ispano (Papa Giovanni XXI))
LC manual cat. (hdg.: Joannes XXI, Pope, d. 1277; usage: Pedro Hispano, Peter of Spain, Pietro Spano)
New Cath. encyc., c1967 (John XXI, Pope (Peter of Spain); b. Petrus Juliani or Petrus Hispanus, Lisbon, between c. 1210 and 1215; his best-known work, the Summulae logicales)