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How strong is Cantor-Bernstein-Schröder?

The Cantor-Schröder-Bernstein theorem (CBS) implies excluded middle in intuitionistic mathematics (INT) and in recursive mathematics (RUSS), since CBS is false ($\neg$CBS is a theorem), and ex falso q …
Franka Waaldijk's user avatar
1 vote
Accepted

Fixed-point property and $T_0$ separation property

Consider in intuitionistic mathematics the example of $A$ being the unit interval $[0,1]$ with the trivial topology $\{\emptyset,[0,1]\}$. Then $A$ is not $T_0$ and yet $A$ still has the fixed-point …
Franka Waaldijk's user avatar
2 votes

Adding nonconstructive disjunction to intuitionistic logic

${\rm\small\color{#808080}{(1st\ answer, also\ see\ updates\ below):}}$ I think the connective $A⅋B$ is not more constructive than existing connectives. You say that a witness for$A⅋B$ consists of two …
Franka Waaldijk's user avatar
4 votes
Accepted

Why is the notion of algorithm a primitive one in Brouwer's intuitionism?

Since the work of Church and Turing (say around 1936), the notion of algorithm is definitely not considered primitive in intuitionism. …
Franka Waaldijk's user avatar
1 vote

Admissibility of Harrop's rule, computationally

The way I understand it is as follows. The most general formula $ (\neg A \; \to \; B \vee C) \;\; \to \;\; ((\neg A \; \to \; B) \vee (\neg A \; \to \; C)) $ is not derivable in IPC, because it …
Franka Waaldijk's user avatar