Class Diary for M351, Spring 2007, Jochen Denzler

Wed Jan 10: Intro: Definition of a group and first examples. Hwk: study Examples and Hwk 3-6 from the Group Zoopark sheet for discussion Fri.
Fri Jan 12: Left neutral and left inverse imply existence and uniqueness of neutral and inverse. Subgroups. More examples. Hwk: 7-10; 8 for grading; due Wed.
Mon Jan 15: MLK DAY
Wed Jan 17: Discussion of group examples; in particular the history of groups as transformation groups. Hwk: 10 for grading due Fri. All others on your own; presentation on Fri as needed -- Definition of rings and fields. Hwk: compare field axioms, as built up from group and ring in class, with field axioms for the real numbers in M300
Fri Jan 19: Hwk discussion. First few examples of a ring. Hwk: begin 11; more in Monday's class. Read book Sec 4.3
Mon Jan 22: More Hwk discussion. Examples of Rings. Hwk 11 due Wed (for discussion and presentationa as needed, minus formal grading). Begin to look at new hwk as well
Wed Jan 24: Discussion Hwk 11. Divisibility, units and notion of gcd defined in commutative ring with identity; primary example is ring of integers. Other examples will follow later. Hwk: 13-15 due Mon (hard deadline) for grading. 12,16,17 due Wed for grading (soft deadline)
Fri Jan 26: Euclidean algorithm in the ring of integers.
Mon Jan 29: Prime numbers in the ring of integers, and general remarks towards generalization of primality / irreducibility to more general rings. Equivalence of primality and irreducibility in Z. Hwk: 19 due Wed (not for grading)
Wed Jan 31: Hwk discussion. Congruence mod n. Outline of construction of the ring Zn
Fri Feb 02: Comments on construction of ring R[i] (hwk 15). Proofs for well-def'dness and ring properties of Zn. Hwk due Wed: 18,21 not graded (presentation as needed); 23+24 graded. 20 pingpong correct by Wed= 1pt EC. Also have a look at 22 and see if you have questions
Mon Feb 05: Direct sum of rings; Chinese Remainder Thm; zero divisors and integral domain defined.
Wed Feb 07: finished Chinese remainder theorem. Uniqueness of prime factor decomposition. Zn is a field if n is prime, and has zero divisors if n is composite. Hwk: 22, 25-27 for presentation as needed. (Note: formerly numbered funnily: 22,25,25,26) Begun: The chinese remainder theorem translated into an isomorphism of rings statement. (with the isomorphism notion yet to be defined during, and motivated by this example).
Fri Feb 09: Ring (and group) homomorphisms and isomorphisms; in particular the example Zm + Zn isomorphic Zmn for gcd(m,n)=1. Euler-phi function introduced.
Mon Feb 12: Discussion of Hwk 25. Another example of a ring isomorphism (from Example + Hwk 36)
Wed Feb 14: Review of Hwk 16. Isomorphism of P(M) with a ring of functions via indicator function Hwk: 31-41 for presentation as needed
Fri Feb 16: More homo-/isomorphism examples. Hwk 40 for grading due Wed
Mon Feb 19: Euler-phi; multiplicativity property and formula. a to the power phi(n) congruent 1 modulo n, provided gcd(a,n)=1. period lengths of fractions in decimal expansion.
Wed Feb 21: Proofs concerning period lengths of 1/p and phi(p). Other uses of modular arithmetic. Hwk: 28-30 for grading due Mon; 44 already by Fri (no grading)
Fri Feb 23: comments on Hwk 44. Ideals; defs and examples.
Mon Feb 26 : Hints for Hwk 29, 46. --- Hwk 28-30 and 46 will be collected for grading on Wed; hard deadline; and solutions posted then, for exam prep use. Ideals are for the purpose of constructing residue class rings.
Wed Feb 28: Construction of residue class ring with proofs. Example Z[i]/(3) started.
Fri Mar 02: EXAM 1
Mon Mar 05: Z[i]/(3) finished. Exam back. Hwk 42,45 for grade asap. 43 for discussion as needed
Wed Mar 07: Overview over polynomial rings: Modular arithmetic helps to decide factorization of polynomials with rational coefficients; therefore we study also polynomials whose coefficients are in rings Z_n. But in those rings, polynomials must be distinguished from polynomial functions. So we require a pure-bred algebra construction of polynomial rings, not relying on polynomial *functions* as in calculus.
Fri Mar 09: Construction of R[[X]] (ring of formal power series)
Mon Mar 19: associativity of . in R[[X]] proved. The polynomial ring R[X] as subring of R[[X]]. Degree defined. Hwk: 50-53 due Fri for grading (hard deadline)
Wed Mar 21: Properties of the degree. The euclidean algorithm for polynomials (started).
Fri Mar 23: Existence of a gcd in R[X] for R a field; begun. R[X] is a principal ideal domain.
Mon Mar 26: Divisibility, units in integral domains; examples; a gcd exists in each PID. Hwk: 47,48 due Fri for grading
Wed Mar 28: Comparison of `pedestrian' and PID proof of existence of gcd. Unique factorization begun.
Fri Mar 30: Proof prime factorization
Mon Apr 02: Uniqueness of prime factorization proof finished. Hwk: 54-57 due Mon (no grading; presentation as needed). 58-60 due Wed after Easter for grading. Sol's will be expected to be careful, 300 style.
Wed Apr 04: Quick overview of book's thm:``If the Integral domain R is a UFD, then R[X] is a UFD, too'' -- We'll do this for Z[X] whose UFD property is still in question b/c it is not a PID. The method relies on knowing that Z is a subring of the field Q, and Q is the samllest field containing Z as a subring. Such a field can be constructed for every integral domain (the book did it, we didn't, yet). So we'll prove the UFD property for Z[X], but the proof is paradigmatic for the book's general case.
Mon Apr 09: Gauss lemma; factorization in Z[X]
Wed Apr 11: Uniqueness of factorization in Z[X]. -- Evaluation homomorphism; roots; polynomials vs polynomial functions.
Fri Apr 13: Irreducibility tests: rational root test; Eisenstein
Mon Apr 16: EXAM 2
Wed Apr 18: Discussion of exam. Factor rings of polynomial rings (review and examples).
Fri Apr 20: More examples for factor rings of polynomial rings. Evaluation
Mon Apr 23: Z_2[X]/(X^2+X+1) is a field. Informally: General construction principle for fields with finitely many (prime power) elements is: Mod out an ideal generated by an irreducible polynomial of degree n from Z_p[X]. (No proofs given). Review on some properties of polynomial rings with this hwk in view: Hwk: 61-65; won't be graded; sol's to be posted soon
Wed Apr 25: R comm ring with 1. When is a factor ring R/I a field? Answer: exactly when I is a maximal ideal. Maximal ideals in poly rings F[X] are exactly those generated by irreducible polynomials.
Fri Apr 27: Q&A; in particular on Euler-phi. Some comments on exam. A quick overview how to construct the quotient field of an integral domain.
Mon Apr 30: STUDY PERIOD. Regular class time will offer a Q&A session.
Wed May 02: FINAL EXAM: 10:15-12:15 scheduled by university policy