Maybe someone familiar with Willard's textbook can help me out. Problem section 23G on pg. 174 is titled piecewise metrizability. The first problem is:

1. If a Tychonoff space $X$ is the union of a locally finite collection of closed, metrizable subspaces, then X is metrizable.

This fact is well-known. It is 4.4.19 in Engelking's book. It also appears in Nagata's seminal metrization paper from 1950. However, the conclusion is true without the assumption that $X$ is Tychonoff. This leads me to think there is a simpler proof under this assumption. If you know of one, please give me an outline or hint.

More interesting is the next problem from Willard:

If a $T_4$ space $X$ is the union of any locally finite collection of metrizable subspaces, then $X$ is metrizable. [Use 15.10]

This is less well-known, because it is clearly not true. Consider the one point compactification of an uncountable discrete space. It is normal and non-metrizable, but is the union of just two metrizable subspaces: one discrete and the other a singleton. The hint refers to the standard "shrinking" theorem for point-countable open covers of normal spaces. So, it seems likely that the word "open" was just accidentally omitted. The following fact follows easily from the first exercise and 15.10:

Fact: If a $T_4$ space $X$ is the union of a locally finite collection of open, metrizable subspaces, then $X$ is metrizable.

Certainly, someone must have noticed this before now. However, I haven't been able to find any reference to, or use of, the above fact in print. Does anyone know of a better citation for this than "Corrected version of exercise 2 in 23G of [W]"?

Thanks in advance,
-Jeff

Refs:

• Willard, Stephen General topology Originally printed 1970 by AddisonWesley; currently in print by Dover Publications (2004)
• Engelking, Ryszard General topology Heldermann Verlag (1989)
• Nagata, Jun-iti On a necessary and sufficient condition of metrizability J. Inst. Polytech. Osaka City Univ. (1950)

PS: Unfortunately, exercises 1 and 2 also appear (without correction) in Patty's more recent textbook, Foundations of Topology, Jones and Bartlett Publishers (2009). They appear to have been copied verbatim from Willard.

Maybe someone familiar with Willard's textbook can help me out. Problem section 23G on pg. 174 is titled piecewise metrizability. The first problem is:

1. If a Tychonoff space $X$ is the union of a locally finite collection of closed, metrizable subspaces, then X is metrizable.

This fact is well-known. It is 4.4.19 in Engelking's book. It also appears in Nagata's seminal metrization paper from 1950. However, the conclusion is true without the assumption that $X$ is Tychonoff. This leads me to think there is a simpler proof under this assumption. If you know of one, please give me an outline or hint.

More interesting is the next problem from Willard:

2. If a $T_4$ space $X$ is the union of any locally finite collection of metrizable subspaces, then $X$ is metrizable. [Use 15.10]

This is less well-known, because it is clearly not true. Consider the one point compactification of an uncountable discrete space. It is normal and non-metrizable, but is the union of just two metrizable subspaces: one discrete and the other a singleton. The hint refers to the standard "shrinking" theorem for point-countable open covers of normal spaces. So, it seems likely that the word "open" was just accidentally omitted. The following fact follows easily from the first exercise and 15.10:

Fact: If a $T_4$ space $X$ is the union of a locally finite collection of open, metrizable subspaces, then $X$ is metrizable.

Certainly, someone must have noticed this before now. However, I haven't been able to find any reference to, or use of, the above fact in print. Does anyone know of a better citation for this than "Corrected version of exercise 2 in 23G of [W]"?

Thanks in advance,
-Jeff

Refs:

• Willard, Stephen General topology Originally printed 1970 by AddisonWesley; currently in print by Dover Publications (2004)
• Engelking, Ryszard General topology Heldermann Verlag (1989)
• Nagata, Jun-iti On a necessary and sufficient condition of metrizability J. Inst. Polytech. Osaka City Univ. (1950)

PS: Unfortunately, exercises 1 and 2 also appear (without correction) in Patty's more recent textbook, Foundations of Topology, Jones and Bartlett Publishers (2009). They appear to have been copied verbatim from Willard.