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Theorem wl-df-3mintru2 34894
Description: Alternative definition of wcad 1608. See df-cad 1609 to learn how it is currently introduced. The only use case so far is being a binary addition primitive for df-sad 15793. If inputs are viewed as binary digits (true is 1, false is 0), the result is whether ordinary binary full addition yields a carry bit. That is what the name df-cad 1609 is derived from: "carry of an addition". Here we stick with this abbreviated form of our notation above, but still use "adder carry" as a shorthand for "at least 2 out of 3" in text.

The core meaning is to check whether at least two of three inputs are true. So, if the first input is true, at least one of the two remaining must be true, else even both. This theorem is the in-between of "at least 1 out of 3", given by triple disjunction df-3or 1085, and "(at least) 3 out of 3", expressed by triple conjunction df-3an 1086.

The notion above can be generalized to other input numbers with other minimum values as follows. Let us introduce informally a logical operation "n-mintru-m" taking n inputs, and requiring at least m of them be true to let the operation itself be true. There now exists a recursive scheme to define it for increasing n, m. We start with the base case n = 0. Here "n-mintru-0" is equivalent to (any sequence of inputs contains at least zero true inputs), the other "0-mintru-m" is for any m > 0 equivalent to , because a sequence of zero inputs never has a positive number of them true. The general case adds a new input 𝜑 to a given sequence of n inputs, and reduces that case for all integers m to that of the smaller sequence by recursion, informally written as:

"(n+1)-mintru-(m+1)" ↔ if-(𝜑, "n-mintru-m" , "n-mintru-(m+1)" )

Our definition here matches "3-mintru-2" with inputs 𝜑, 𝜓 and 𝜒. Starting from the base cases we find after simplifications: "2-mintru-2" (𝜓, 𝜒) ↔ (𝜓𝜒) (wl-2mintru2 34901), and "2-mintru-1" (𝜓, 𝜒) ↔ (𝜓𝜒) (wl-2mintru1 34900). Plugging these expressions into the formula above for n = 3, m = 2 yields exactly our definition here. (Contributed by Wolf Lammen, 2-May-2024.)

Assertion
Ref Expression
wl-df-3mintru2 (cadd(𝜑, 𝜓, 𝜒) ↔ if-(𝜑, (𝜓𝜒), (𝜓𝜒)))

Proof of Theorem wl-df-3mintru2
StepHypRef Expression
1 cadrot 1616 . 2 (cadd(𝜑, 𝜓, 𝜒) ↔ cadd(𝜓, 𝜒, 𝜑))
2 cadifp 1620 . 2 (cadd(𝜓, 𝜒, 𝜑) ↔ if-(𝜑, (𝜓𝜒), (𝜓𝜒)))
31, 2bitri 278 1 (cadd(𝜑, 𝜓, 𝜒) ↔ if-(𝜑, (𝜓𝜒), (𝜓𝜒)))
Colors of variables: wff setvar class
Syntax hints:  wb 209  wa 399  wo 844  if-wif 1058  caddwcad 1608
This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8
This theorem depends on definitions:  df-bi 210  df-an 400  df-or 845  df-ifp 1059  df-3or 1085  df-3an 1086  df-xor 1503  df-cad 1609
This theorem is referenced by:  wl-df2-3mintru2  34895  wl-df3-3mintru2  34896  wl-df3maxtru1  34902
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