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Péter Komjáth
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Two more solutions.

  1. Start with a model with a Suslin tree $(T,\leq)$ in it (force it or use L). Let $(P,\leq)$ be the Cohen forcing adding any number of reals. We claim that $(T,\leq)$ remains Suslin after forcing with $(P,\leq)$. Assume that $p$ forces that $A$ is an uncountable antichain in $T$. There are conditions $p_\alpha\leq p$ and distinct elements $t_\alpha\in T$ such that $p_\alpha$ forces that $t_\alpha\in A$. An easy argument (using the delta system lemma) shows that there is an uncountable set $Z$ such that any finite subfamily of $\{p_\alpha:\alpha\in Z\}$ has a common lower bound. Then $\{t_\alpha:\alpha\in Z\}$ is an antichain, as if $\alpha\neq\beta$ are in $Z$, then some $p'\leq p_\alpha,p_\beta$ forces that $t_\alpha$, $t_\beta$ are both in $A$. Contradiction to the fact that $T$ is Suslin in the ground model.

  2. In Jech's above mentioned book, there is a forcing adding a Suslin tree with finite conditions (in the exercises). Force with it over a model of non-CH.

In K. Devlin: $\aleph_1$ trees, Annals of Math. Logic, 13(1978), 267-330, all combinations of CH, ST (there exists a Suslin tree) and KT (there exists a Kurepa tree) and their negations are considered.

Two more solutions.

  1. Start with a model with a Suslin tree $(T,\leq)$ in it (force it or use L). Let $(P,\leq)$ be the Cohen forcing adding any number of reals. We claim that $(T,\leq)$ remains Suslin after forcing with $(P,\leq)$. Assume that $p$ forces that $A$ is an uncountable antichain in $T$. There are conditions $p_\alpha\leq p$ and distinct elements $t_\alpha\in T$ such that $p_\alpha$ forces that $t_\alpha\in A$. An easy argument (using the delta system lemma) shows that there is an uncountable set $Z$ such that any finite subfamily of $\{p_\alpha:\alpha\in Z\}$ has a common lower bound. Then $\{t_\alpha:\alpha\in Z\}$ is an antichain, as if $\alpha\neq\beta$ are in $Z$, then some $p'\leq p_\alpha,p_\beta$ forces that $t_\alpha$, $t_\beta$ are both in $A$. Contradiction to the fact that $T$ is Suslin in the ground model.

  2. In Jech's above mentioned book, there is a forcing adding a Suslin tree with finite conditions (in the exercises). Force with it over a model of non-CH.

Two more solutions.

  1. Start with a model with a Suslin tree $(T,\leq)$ in it (force it or use L). Let $(P,\leq)$ be the Cohen forcing adding any number of reals. We claim that $(T,\leq)$ remains Suslin after forcing with $(P,\leq)$. Assume that $p$ forces that $A$ is an uncountable antichain in $T$. There are conditions $p_\alpha\leq p$ and distinct elements $t_\alpha\in T$ such that $p_\alpha$ forces that $t_\alpha\in A$. An easy argument (using the delta system lemma) shows that there is an uncountable set $Z$ such that any finite subfamily of $\{p_\alpha:\alpha\in Z\}$ has a common lower bound. Then $\{t_\alpha:\alpha\in Z\}$ is an antichain, as if $\alpha\neq\beta$ are in $Z$, then some $p'\leq p_\alpha,p_\beta$ forces that $t_\alpha$, $t_\beta$ are both in $A$. Contradiction to the fact that $T$ is Suslin in the ground model.

  2. In Jech's above mentioned book, there is a forcing adding a Suslin tree with finite conditions (in the exercises). Force with it over a model of non-CH.

In K. Devlin: $\aleph_1$ trees, Annals of Math. Logic, 13(1978), 267-330, all combinations of CH, ST (there exists a Suslin tree) and KT (there exists a Kurepa tree) and their negations are considered.

Source Link
Péter Komjáth
  • 7.1k
  • 2
  • 37
  • 43

Two more solutions.

  1. Start with a model with a Suslin tree $(T,\leq)$ in it (force it or use L). Let $(P,\leq)$ be the Cohen forcing adding any number of reals. We claim that $(T,\leq)$ remains Suslin after forcing with $(P,\leq)$. Assume that $p$ forces that $A$ is an uncountable antichain in $T$. There are conditions $p_\alpha\leq p$ and distinct elements $t_\alpha\in T$ such that $p_\alpha$ forces that $t_\alpha\in A$. An easy argument (using the delta system lemma) shows that there is an uncountable set $Z$ such that any finite subfamily of $\{p_\alpha:\alpha\in Z\}$ has a common lower bound. Then $\{t_\alpha:\alpha\in Z\}$ is an antichain, as if $\alpha\neq\beta$ are in $Z$, then some $p'\leq p_\alpha,p_\beta$ forces that $t_\alpha$, $t_\beta$ are both in $A$. Contradiction to the fact that $T$ is Suslin in the ground model.

  2. In Jech's above mentioned book, there is a forcing adding a Suslin tree with finite conditions (in the exercises). Force with it over a model of non-CH.