I was wondering if there is a list of the most active branches of mathematics?
If MathOverflow is a representative sample, then algebraic geometry is by far the most popular. Is this the case?
I was wondering if there is a list of the most active branches of mathematics? If MathOverflow is a representative sample, then algebraic geometry is by far the most popular. Is this the case? 


As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.If this question can be reworded to fit the rules in the help center, please edit the question.
Sorry to add to the noise, but here it goes. With a little scriptfu (and emacs, of course!) I retrieved the data from MSC corresponding to the last ten years in each of the Primary Classifications. Annoyingly the AMS changed their subject classification scheme recently, so that the numbers I queried were interpreted as MSC2010, whereas the papers are published from the year 2000. 43465 35 Partial differential equations 38151 62 Statistics 35994 81 Quantum theory 35633 68 Computer science 34474 65 Numerical analysis 28593 05 Combinatorics 28296 90 Operations research, mathematical programming 26406 34 Ordinary differential equations 26192 60 Probability theory and stochastic processes 23879 93 Systems theory; control 22361 11 Number theory 21689 76 Fluid mechanics 20787 91 Game theory, economics, social and behavioral sciences 19440 37 Dynamical systems and ergodic theory 18425 83 Relativity and gravitational theory 17323 94 Information and communication, circuits 17247 53 Differential geometry 16465 47 Operator theory 16134 03 Mathematical logic and foundations 15408 20 Group theory and generalizations 14225 92 Biology and other natural sciences 14051 82 Statistical mechanics, structure of matter 13663 46 Functional analysis 12894 74 Mechanics of deformable solids 11241 14 Algebraic geometry 10237 49 Calculus of variations and optimal control; optimization 10215 30 Functions of a complex variable 10154 16 Associative rings and algebras 9801 01 History and biography 9781 54 General Topology 8014 42 Fourier analysis 7103 58 Global analysis, analysis on manifolds 6780 15 Linear and multilinear algebra; matrix theory 6410 70 Mechanics of particles and systems 6359 32 Several complex variables and analytic spaces 6348 57 Manifolds and cell complexes 6185 41 Approximations and expansions 5935 39 Difference and functional equations 5684 26 Real functions 5349 17 Nonassociative rings and algebras 5226 13 Commutative rings and algebras 4840 78 Optics, electromagnetic theory 4439 52 Convex and discrete geometry 4418 33 Special functions 4350 00 General 3818 06 Order, lattices, ordered algebraic structures 3511 28 Measure and integration 3295 51 Geometry 2948 22 Topological groups, Lie groups 2944 55 Algebraic topology 2538 86 Geophysics 2089 45 Integral equations 2052 18 Category theory; homological algebra 1679 80 Classical thermodynamics, heat transfer 1523 31 Potential theory 1444 43 Abstract harmonic analysis 1343 12 Field theory and polynomials 1161 40 Sequences, series, summability 1108 08 General algebraic systems 898 44 Integral transforms, operational calculus 775 19 Ktheory 534 85 Astronomy and astrophysics Usual disclaimers apply. In particular, before concluding that nobody works in astrophysics, go and check the submission statistics for Let me reiterate that I do not believe for a second that this data allows one to conclude anything of value about mathematics, just perhaps about mathematicians :) Added (incorporating Gerald Edgar's summary in the comment below) This is the summary of "pure maths" defined as classifications 0060, with a total of 0008 Logic and Combinatorics 63804 15.49% 1120 Algebra and Number Theory 80689 19.59% 2249,60 Analysis and Probability 216252 52.50% 5158 Geometry and Topology 51157 12.42% 


For those who have time to do some coding, the AMS releases tables of how many papers in MathSciNet land in each of the MSC subjects. This should be a more representative sampling of mathematical publications than the arXiv. Unfortunately, the format is a list of every paper, its year of publication, and which classifications it used, so it is not obvious to a human which subjects are the most popular. For those who don't have the energy to create our own table, David Rusin has a chart where the area of each MSC subject is proportional to the number of publications in that filed from 19802000. The classification is too fine to easily answer questions like "Is analysis more popular than algebra" and the time period is not quite what we want. But one can immediately see that any one of Statistics (62), Probability and Stochastic Processes (60), Numerical Analysis (65) and PDEs (35) all dwarf Algebraic Geometry (14), Category Theory (18) and even Number Theory (11). 


The word "current" is my getout on this! Here's the data from the arXiv for the month of december:
(yeah, yeah, I know  skewed results since it came from the arXiv ... yawn, think of a new complaint, please.) 


I think it is a little bit anachronistic to divide mathematical disciplines and search for the most "active" one. The modern tendency (justified by the major achievements of contemporary mathematics) is to ignore the "barriers" between the different fields and become truly interdisciplinary. 


"Fashionable" is so subjective that it should be avoided here, I think. On the other hand, it is very natural to wonder about which subject areas  as represented, say, in the 2010 AMS Mathematics Subject Classification  are the most popular as measured e.g. in terms of total papers published in the last ten years or the total number of mathematicians who have published in this area. I'm not about to try to implement a computer search to answer this question, but it seems likely that someone else has already done so. I will predict an answer though: algebraic geometry is not the most popular research area in any quantitative sense. (Others have asked why algebraic geometry is so prevalent on MO and the most convincing answer seems to be that the founders of MO are mostly algebraic geometers and mathematicians in closely related areas.) I would be willing to bet that, as has been the case for at least one hundred years, more papers are published in analysis than in any other area. 


I'm skeptical that this question can be asked and answered in a meaningful manner. Do we really want to know which area of mathematics produces, say, the most papers? Or even the most citations? What might be more meaningful (but maybe not) is which fields get the most funding from NSF. 


Given the hint from David, here's what 30s coding produces:
As it's only 30s, I'll leave it to others to fill in the data about which area is which MSC. (Community wikied so that others can easily do that). Oh, it's the 2007 data (most recent) by the way. (yeah, yeah, I know  skewed results since it came from the MSC ... yawn, think of a new complaint, please.) 

