Update URLs

docs/developer/gsoc-ideas.html

@@ -270,7 +270,7 @@ <h2 id="get-started">How to get started: do a case study</h2>
 understand the Checker Framework from the user point of view, which will
 not happen until you have completed a case study on an open-source program.)
 You may discuss your ideas with us by sending mail
-to <a href="https://groups.google.com/forum/#!forum/checker-framework-gsoc">checker-framework-gsoc@googlegroups.com</a>.
+to <a href="https://groups.google.com/g/checker-framework-gsoc">checker-framework-gsoc@googlegroups.com</a>.
 </p>
 
 
@@ -1121,7 +1121,7 @@ <h2 id="nullness-bug-detector">Nullness bug detector</h2>
 <p>
 Verifying a program to be free of errors can be a daunting task.  When
 starting out, a user may be more interested in
-<a href="https://checkerframework.org/manual/#other-tools">bug-finding</a>
+<a href="https://checkerframework.org/manual/#faq-other-tools-section">bug-finding</a>
 than verification.  The goal of this project is to create a nullness bug
 detector that uses the powerful analysis of the Checker Framework and its
 Nullness Checker, but omits some of its more confusing or expensive
@@ -1197,7 +1197,7 @@ <h2 id="index-errors">Improving error messages</h2>
 <p>
 Compiler writers have come to realize that clarity of error
 messages is as important as the speed of the executable
-(<a href="http://www.brettbecker.com/wp-content/uploads/2016/06/Becker-Effective-2016-SIGCSE.pdf">1</a>, <a href="https://www.mville.edu/sites/default/files/p53-munson_1.pdf">2</a>,
+(<a href="https://www.brettbecker.com/wp-content/uploads/2016/06/Becker-Effective-2016-SIGCSE.pdf">1</a>, <a href="https://www.mville.edu/sites/default/files/p53-munson_1.pdf">2</a>,
 <a href="http://se.ethz.ch/~meyer/publications/teaching/compiler-errors.pdf">3</a>,
 <a href="http://static.barik.net/barik/publications/icse2017/PID4655707.pdf">4</a>).  This is especially true when the language or type system has rich features.
 </p>
@@ -1369,7 +1369,7 @@ <h2 id="Purity_analysis">Purity (side effect) analysis</h2>
 <p>
 Perhaps one of these existing side effect analyses could be used:
 <a href="https://github.com/Sable/soot/wiki/Using-Side-Effect-Attributes">Soot</a>,
-<a href="https://pdfs.semanticscholar.org/d2bd/29e20c99a10ab9d0c5ecf3acaf15606407d1.pdf">Geffken</a>.
+<a href="https://www.semanticscholar.org/paper/Precise-Interprocedural-Side-Effect-Analysis-Report-Geffken-Saffrich/d2bd29e20c99a10ab9d0c5ecf3acaf15606407d1?p2df">Geffken</a>.
 </p>
 
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