Definition df-frecs

Description: This is the definition for the founded recursion generator. Similar to df-wrecs and df-recs , it is a direct definition form of normally recursive relationships. Unlike the former two definitions, it only requires a founded set-like relationship for its properties, not a well-founded relationship. When this relationship is also a partial ordering, the proof does not use the Axiom of Infinity, but it requires Infinity when the order is not partial. We develop the theorems twice, once with partial ordering and once without. (Contributed by Scott Fenton, 23-Dec-2021)

	Ref	Expression
Assertion	df-frecs	$$⊢\mathrm{frecs}\left(R,A,F\right)=\bigcup \left\{f|\exists x\left(fFnx\wedge \left(x\subseteq A\wedge \forall y\in xPred\left(R,A,y\right)\subseteq x\right)\wedge \forall y\in xf\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)\right)\right\}$$

Detailed syntax breakdown

Step	Hyp	Ref	Expression
0		cR	$$classR$$
1		cA	$$classA$$
2		cF	$$classF$$
3	1 0 2	cfrecs	$$class\mathrm{frecs}\left(R,A,F\right)$$
4		vf	$$setvarf$$
5		vx	$$setvarx$$
6	4	cv	$$setvarf$$
7	5	cv	$$setvarx$$
8	6 7	wfn	$$wfffFnx$$
9	7 1	wss	$$wffx\subseteq A$$
10		vy	$$setvary$$
11	10	cv	$$setvary$$
12	1 0 11	cpred	$$classPred\left(R,A,y\right)$$
13	12 7	wss	$$wffPred\left(R,A,y\right)\subseteq x$$
14	13 10 7	wral	$$wff\forall y\in xPred\left(R,A,y\right)\subseteq x$$
15	9 14	wa	$$wff\left(x\subseteq A\wedge \forall y\in xPred\left(R,A,y\right)\subseteq x\right)$$
16	11 6	cfv	$$classf\left(y\right)$$
17	6 12	cres	$$class\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)$$
18	11 17 2	co	$$class\left(yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)\right)$$
19	16 18	wceq	$$wfff\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)$$
20	19 10 7	wral	$$wff\forall y\in xf\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)$$
21	8 15 20	w3a	$$wff\left(fFnx\wedge \left(x\subseteq A\wedge \forall y\in xPred\left(R,A,y\right)\subseteq x\right)\wedge \forall y\in xf\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)\right)$$
22	21 5	wex	$$wff\exists x\left(fFnx\wedge \left(x\subseteq A\wedge \forall y\in xPred\left(R,A,y\right)\subseteq x\right)\wedge \forall y\in xf\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)\right)$$
23	22 4	cab	$$class\left\{f|\exists x\left(fFnx\wedge \left(x\subseteq A\wedge \forall y\in xPred\left(R,A,y\right)\subseteq x\right)\wedge \forall y\in xf\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)\right)\right\}$$
24	23	cuni	$$class\bigcup \left\{f|\exists x\left(fFnx\wedge \left(x\subseteq A\wedge \forall y\in xPred\left(R,A,y\right)\subseteq x\right)\wedge \forall y\in xf\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)\right)\right\}$$
25	3 24	wceq	$$wff\mathrm{frecs}\left(R,A,F\right)=\bigcup \left\{f|\exists x\left(fFnx\wedge \left(x\subseteq A\wedge \forall y\in xPred\left(R,A,y\right)\subseteq x\right)\wedge \forall y\in xf\left(y\right)=yF\left({f\upharpoonright }_{Pred\left(R,A,y\right)}\right)\right)\right\}$$