Notices of American Mathematical Society

https://goo.gl/XVvXTz

Or generally (just replace the 201603 with yyyymm):

http://www.ams.org/journals/notices/201603/201603-full-issue.pdf

Or this URL as well:

http://www.ams.org/notices/201502/201502-full-issue.pdf

Polish Mathematical Olympiad 2016/17 - Mathematics Meta Stack Exchange

https://math.meta.stackexchange.com/questions/25020/polish-mathematical-olympiad-2016-17?rq=1

SIAM: Math Matters, Apply It!

https://www.siam.org/careers/matters.php

Try these problems from historical Tripos

A trip into Mathematical Tripos through its history

Looking at this:

http://maths-obsession.blogspot.sg/2015/05/searching-for-mathematical-tripos.html

Many links are outdated, so perhaps here are some updated links:

https://www.maths.cam.ac.uk/undergrad/pastpapers

And these are the pre-1878 era:

https://www.ocf.berkeley.edu/~lekheng/tripos/math-tripos-pre1878.pdf

And lots of others:

https://www.ocf.berkeley.edu/~lekheng/tripos/

https://tartarus.org/gareth/maths/tripos/

https://www.maths.cam.ac.uk/past-examination-papers

https://www.maths.cam.ac.uk/undergradnst/pastpapers

Good Olympics Questions (Computing)

http://wcipeg.com/problems   (from China)

https://noi.ph/past-problems/  (Phillipines)

https://github.com/ranaldmiao/sg_noi_archive (singapore)

http://codeforces.com/problemset

Putnam problems

https://web.archive.org/web/20080124125203/www.kalva.demon.co.uk/putnam.html

Non-linear programming

http://www.math.ku.dk/~moller/undervisning/MASO2010/kuhntucker1950.pdf

http://math.bme.hu/~bog/GlobOpt/Chapter5.pdf

http://orfe.princeton.edu/~rvdb/tex/INFORMSDenver/tutorial3.pdf

http://web.mit.edu/15.053/www/AMP-Chapter-13.pdf

http://www.ai7.uni-bayreuth.de/eolss.pdf

http://www.sce.carleton.ca/faculty/chinneck/po/Chapter16.pdf

http://cepac.cheme.cmu.edu/pasilectures/biegler/BieglerLecture.pdf

Non-linear programming

https://wps.prenhall.com/wps/media/objects/14127/14466190/online_modules/taylor_ims11_module_D.pdf 

https://web.stanford.edu/class/msande310/310trialtext.pdf 

http://www.sce.carleton.ca/faculty/chinneck/po/Chapter18.pdf 

https://wps.prenhall.com/wps/media/objects/14127/14466190/online_modules/taylor_ims11_module_D.pdf 

Statistics and Probability Theory

https://people.math.ethz.ch/~wueth/Positions/2013_Bernoulli.pdf

http://www.math.umd.edu/~slud/s470/BookChaps/01Book.pdf

http://santos.ee.ntu.edu.tw/statcomm2011/1_Probability_Theory_&_Mathematical_Statistics_(classnote).pdf  

http://www.stat.ncsu.edu/people/schwartzman/courses/ST521/lect07-DiscreteDistr_handout.pdf 

http://www.stat.cmu.edu/~larry/=stat705/Lecture8.pdf 

http://www.stat.umn.edu/geyer/f08/5101/notes/n.pdf 

https://www.mpimet.mpg.de/fileadmin/staff/vonstorchjin-song/vls-s/s3.pdf 

http://www.stats.ox.ac.uk/~reinert/stattheory/theoryshort09.pdf 

http://www.math.harvard.edu/~ctm/papers/home/text/class/harvard/154/course/course.pdf 

http://math.arizona.edu/~jwatkins/notests.pdf

http://www.math.umd.edu/~slud/s470/BookChaps/ActuChp1.pdf  

http://www.math.umd.edu/~slud/s470/BookChaps/ActuChp2.pdf 

http://www.math.umd.edu/~slud/s470/BookChaps/ActuChp3.pdf 

http://www.math.umd.edu/~slud/s470/BookChaps/Chp45.pdf 

http://www.math.umd.edu/~slud/s470/BookChaps/Chp6.pdf

Hilbert's Ten Problem

https://www.math.washington.edu/~morrow/336_15/papers/andrew.pdf

http://math.stanford.edu/~lekheng/flt/wiles.pdf   

https://en.wikipedia.org/wiki/Diophantine_equation

https://en.wikipedia.org/wiki/Goldbach's_conjecture

http://sweet.ua.pt/tos/goldbach.html

https://arxiv.org/pdf/math/0309103.pdf

MAA Contest Problem Book IV:

http://files.eric.ed.gov/fulltext/ED239856.pdf