We have the following indirect implication of form equivalence classes:
			
| Implication | Reference | 
|---|---|
| 258 \(\Rightarrow\) 255 | Variations of Zorn's lemma, principles of cofinality, and Hausdorff's maximal principle, Part I and II, Harper,  J. 1976, Notre Dame J. Formal Logic | 
| 255 \(\Rightarrow\) 260 | Variations of Zorn's lemma, principles of cofinality, and Hausdorff's maximal principle, Part I and II, Harper,  J. 1976, Notre Dame J. Formal Logic | 
| 260 \(\Rightarrow\) 40 | Variations of Zorn's lemma, principles of cofinality, and Hausdorff's maximal principle, Part I and II, Harper,  J. 1976, Notre Dame J. Formal Logic | 
| 40 \(\Rightarrow\) 231 | Abzählbarkeit und Wohlordenbarkeit, Felgner,  U. 1974, Comment. Math. Helv. | 
| 231 \(\Rightarrow\) 294 | clear | 
Here are the links and statements of the form equivalence classes referenced above:
| Howard-Rubin Number | Statement | 
|---|---|
| 258: | \(Z(D,L)\): Every directed relation \((X,R)\) in which linearly ordered subsets have upper bounds, has a maximal element. | 
| 255: | \(Z(D,R)\): Every directed relation \((P,R)\) in which every ramified subset \(A\) has an upper bound, has a maximal element. | 
| 260: | \(Z(TR\&C,P)\): If \((X,R)\) is a transitive and connected relation in which every partially ordered subset has an upper bound, then \((X,R)\) has a maximal element. | 
| 40: | \(C(WO,\infty)\): Every well orderable set of non-empty sets has a choice function. Moore, G. [1982], p 325. | 
| 231: | \(UT(WO,WO,WO)\): The union of a well ordered collection of well orderable sets is well orderable. | 
| 294: | Every linearly ordered \(W\)-set is well orderable. | 
Comment: