[texhax] \noalign{\nobreak} seems not work!

Ian Collier imc at comlab.ox.ac.uk
Mon Aug 11 13:44:12 CEST 2003

Took a while to get back to this, but to round things off...

Rodolfo Medina wrote:
>In the TeXbook, page 193, it is said that the command
>after the \cr in \eqalignno
>will prohibit a page break after that line.

Then doug waud wrote:
>           Now, the log output indicated a penalty of about 100 without
>the an inserted nobreak (I did not chase down where this came from; I
>suspect it would be found in the definition of \noeqalignno)

and I observed that \eqalignno adds an \interdisplaylinepenalty between
adjacent lines.  The default value of \interdisplaylinepenalty just
happens to be 100.

Then I said:
>I don't have the TeXbook with me at the moment so I don't know how it
>explains the above - but looking up \interdisplaylinepenalty may help.

Well having checked my copy of the TeXbook, it doesn't actually make the
statement above on page 193.  It says:

  In fact you can /force/ a page break after a particular line
  if you insert `\noalign{\break}' after the \cr for that line.
  You can prohibit /all/ breaks in an \eqalignno if you set
  \interdisplaylinepenalty=10000; or you can enclose the whole
  works in a \vbox:

Possibly this was modified in an earlier version of plain.tex and
the book was updated to reflect that.  My copy is the seventeenth
printing, revised January 1990.  If not then the original poster
must just have misread what it says.
---- Ian Collier : imc at comlab.ox.ac.uk : WWW page below
------ http://users.comlab.ox.ac.uk/ian.collier/imc.shtml

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