Internet Resources

Here I will keep listing useful “free mathematics websites” (no registration required to enjoy):

1.  Olympiad Problems

• Mathematical Reflections- Online Free Journal : https://www.awesomemath.org/mathematical-reflections/
• Kalva – Olympiad Questions Archive : http://mks.mff.cuni.cz/kalva/
• Putnam Archive: http://kskedlaya.org/putnam-archive/
• Crux Journal : http://cms.math.ca/crux/
• Math Forum – Problems and Puzzles:  http://mathforum.org/problems_puzzles_landing.html
• Iowa State University – Problem of the week: http://orion.math.iastate.edu/ehjohnst/PoW/PoW.html
• Purdue – Problem of the Week: https://www.math.purdue.edu/pow/index
• Bilkent University – Math problem of the month: http://www.fen.bilkent.edu.tr/~cvmath/Problem/problem.htm
• Sobre todo, Matemáticas (Spanish) : https://matemelga.wordpress.com/
• ISI Banglore – Interesting Mathematics Exercises : https://statmathbc.wordpress.com/
• MathsChallenge.net : http://mathschallenge.net/ 
• NRICH : http://nrich.maths.org/students

2.  Recreational Mathematics (Puzzles and Games)

• The Sphinx Collection : http://www.lchr.org/a/36/q5/collection.html
• Alphametic Puzzles: http://www.tkcs-collins.com/truman/alphamet/alphamet.shtml
• Numbrcise : http://www.numbrcise.com
• Puzzlebomb : http://www.puzzlebomb.co.uk/
• Puzzle Collections by Matthew Scroggs : http://www.mscroggs.co.uk/puzzles/
• Tanya Khovanova’s Home Page : http://www.tanyakhovanova.com/
• Number Recreations by Shyam Sunder Gupta : http://www.shyamsundergupta.com
• The Prime Puzzles & Problems Connection by Carlos Rivera : http://www.primepuzzles.net/
• Paper Folding : www.paperfolding.com
• Cut the Knot : www.cut-the-knot.org
• Wild Maths : http://wild.maths.org/
• Jeff Weeks’ Topology and Geometry Software : http://geometrygames.org
• Alex Bellos’s Monday puzzle: https://www.theguardian.com/science/series/alex-bellos-monday-puzzle
• Pradeep Mutalik’s Insights Puzzle: https://www.quantamagazine.org/tag/insights-puzzle/

3.  Online Textbooks

• Mathigon – World of Mathematics : http://world.mathigon.org/
• Abstract Algebra – Theory and Applications by Tom Judson :  http://abstract.ups.edu/aata/aata.html
• Number Theory – In Context and Interactive by Karl-Dieter Crisman : http://www.math-cs.gordon.edu/~kcrisman/mat338/NTIC.html
• A First Course in Linear Algebra by Robert A. Beezer : http://linear.ups.edu/html/fcla.html
• Introduction to Probability, Statistics, and Random Processes by Hossein Pishro-Nik : https://www.probabilitycourse.com/
• Probability Theory and Mathematical Statistics by PennState Eberly College of Science (STAT 414/415) : https://onlinecourses.science.psu.edu/stat414/ 
• Class Field Theory by Chao Li : http://www.math.harvard.edu/~chaoli/doc/ClassFieldTheory.html
• Stable Homotopy Theory by Urs Schreiber : https://ncatlab.org/nlab/show/Introduction+to+Stable+homotopy+theory

4.  Online Magazines

• Bhāvanā : http://bhavana.timc.org.in/
• Recreational Mathematics Magazine : http://rmm.ludus-opuscula.org/
• At Right Angles : teachersofindia.org/en/periodicals/at-right-angles
• Plus : https://plus.maths.org/content/
• Chalkdust : http://chalkdustmagazine.com/
• Quanta : https://www.quantamagazine.org/category/mathematics-2/
• Gonit Sora : http://gonitsora.com/
• Quantum : http://www.nsta.org/publications/quantum.aspx

5.  Encyclopedias

• The On-Line Encyclopedia of Integer Sequences : https://oeis.org/
• Encyclopedia of Triangle Centers :  http://faculty.evansville.edu/ck6/encyclopedia/ETC.html
• Encyclopedia of Quadri-Figures :  http://chrisvantienhoven.nl/index.php/9-mathematics/encyclopedia-of-quadri-figures/3-encyclopedia-of-quadri-figures
• Number Gossip : http://www.numbergossip.com/
• The Number Empire : http://www.numberempire.com/
• What’s Special About This Number? :  http://www2.stetson.edu/~efriedma/numbers.html
• MathPages : http://mathpages.com/
• Subject Wikis : http://subwiki.org/wiki/Main_Page
• Wolfram Mathworld : http://mathworld.wolfram.com
• Visual Calculus Portal: http://archives.math.utk.edu/visual.calculus/
• Random (formerly, Virtual Laboratories in Probability and Statistics) : http://www.randomservices.org/stat/index.html
• Fermat’s Last Theorem: http://shayfam.com/David/flt/index.htm
• The Geometry Junkyard: https://www.ics.uci.edu/~eppstein/junkyard/

6.  LaTeX Tools

• Comprehensive TeX Archive Network: https://www.ctan.org/?lang=en
• mathURL : http://mathurl.com/
• TeX for GMail by Valery Alexeev : http://alexeev.org/gmailtex.html
• Detexify by Daniel Kirsch : http://detexify.kirelabs.org/classify.html
• LaTeX to WordPress: https://lucatrevisan.wordpress.com/latex-to-wordpress/

7.  Entertainment with Mathematics

• Imaginary : http://imaginary.org/
• Numberphile : http://www.numberphile.com/
• Vi Hart : http://vihart.com/
• Stand-up Maths : http://standupmaths.com/
• SingingBanana : http://singingbanana.com/
• Mathologer :  https://www.youtube.com/channel/UC1_uAIS3r8Vu6JjXWvastJg
• 3blue1brown : http://www.3blue1brown.com/
• PBS Infinite Series : https://www.youtube.com/channel/UCs4aHmggTfFrpkPcWSaBN9g
• Mathematics in Movies: http://www.math.harvard.edu/~knill/mathmovies/index.html
• Maths Masters: http://www.qedcat.com/

8.  Collected Works of Mathematicians

• Srinivasa Ramanujan Iyenger : (1) http://www.imsc.res.in/~rao/ramanujan/collectedindex.html       (2) http://ramanujan.sirinudi.org/index.html
• Kurt Mahler :  https://carma.newcastle.edu.au/mahler/collected.html
• Paul Erdős : http://www.renyi.hu/~p_erdos/Erdos.html
• Lars Ahlfors : http://oasis.lib.harvard.edu/oasis/deliver/deepLink?_collection=oasis&uniqueId=hua16002
• Alexander Grothendieck : https://webusers.imj-prg.fr/~leila.schneps/grothendieckcircle/mathtexts.php

9. Open Problem Repositories

• CrowdMath : www.artofproblemsolving.com/polymath
• Open Problem Garden : http://www.openproblemgarden.org/
• Polymath : http://polymathprojects.org/

[Last Edited:15 May 2018]