WITH X(Pnbr,Cnbr) AS
(SELECT 0, 1 FROM dual
 UNION ALL
 SELECT X.Cnbr, X.Pnbr+X.Cnbr FROM X
 WHERE X.Cnbr<50
)
 SELECT * FROM X

ORA-32044: cycle detected while executing recursive WITH query

PNBR  CNBR
0   , 1
1   , 1 + 0

WITH X(iter,Pnbr,Cnbr) AS
(SELECT 1,0, 1 FROM dual
 UNION ALL
 SELECT iter + rownum, X.Cnbr, X.Pnbr+X.Cnbr FROM X
 WHERE iter < 50
)
 SELECT iter,Pnbr FROM X;

WITH X(Pnbr,Cnbr) AS
(SELECT 0, 1 FROM dual
 UNION ALL
 SELECT X.Cnbr, X.Pnbr+X.Cnbr FROM X
 WHERE X.Cnbr<50
)
cycle Pnbr, Cnbr set cycle to 'Y' default 'N'    ----
SELECT Pnbr, Cnbr FROM X