Let R be a ring with unit. The Bass Nil groups NmKi(R) are introduced by Bass[1] in order to investigate the relation between Ki(R[x1, …, xm]) and Ki(R). For any i∈
Theorem A (See Refs. [5-9]). Let R be a ring. For any i, m∈
Let p be a prime number. In some cases, NKi(R) are abelian p-groups[8]. However, the exponents of these abelian p-groups are not completely determined. For example, the exponents of NK0(
Let
$ \begin{array}{*{20}{c}} {{K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]\left[ {{x_1}, \cdots ,{x_m}} \right]} \right) \cong {{\left( {1 + N} \right)}^m}{K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]} \right) = } \\ {\mathop \oplus \limits_{i = 1}^m \left( {\begin{array}{*{20}{c}} m \\ i \end{array}} \right){N^i}{K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]} \right).} \end{array} $ |
In Ref. [12], Juan-Pineda showed the non-finiteness of NK2(
Let
Lemma 1.1 NK2(
Proof Observe that
$ \begin{array}{*{20}{c}} {0 \to {K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]\left[ x \right],J\left[ x \right]} \right) \to {K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]\left[ x \right]} \right)} \\ { \to {K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]\left[ x \right]/J\left[ x \right]} \right) = 0 \to 0,} \end{array} $ |
$ \begin{array}{*{20}{c}} {0 \to N{K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]} \right) \to {K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]\left[ x \right]} \right) \to } \\ {{K_2}\left( {\mathbb{F}\left[ {{C_{{p^n}}}} \right]} \right) = 0 \to 0.} \end{array} $ |
Let I=(t1pn) be a proper ideal in the polynomial ring
$ \mathbb{F}\left[ {{C_{{p^n}}}} \right] \cong \mathbb{F}\left[ {{t_1}} \right]/\left( {t_1^{{p^n}}} \right), $ |
$ \mathbb{F}\left[ {{C_{{p^n}}}} \right]\left[ {{x_1}, \cdots ,{x_m}} \right] \cong \mathbb{F}\left[ {{t_1}, \cdots ,{t_{m + 1}}} \right]/I, $ |
via σ-1 |→t1 and xi |→ti+1. Let A=
Lemma 1.2 Let 1≤t≤n-1 and 1≤h < p be integers. If pn-t-1 < k≤pn-t,
Proof If k=1, the computation is easy. Suppose pn-t-1+1≤k≤pn-t, the result follows from the inequalities
$ {p^{t - 1}} < \frac{{{p^n}}}{{{p^{n - t + 1}} - h}} \le \frac{{{p^n}}}{{pk - h}} \le \frac{{{p^n}}}{{{p^{n - t}} + p - h}} < {p^t}, $ |
$ {p^{t - 1}} = \frac{{{p^n}}}{{{p^{n - t + 1}}}} < \frac{{{p^n} + 1}}{{pk}} \le \frac{{{p^n} + 1}}{{{p^{n - t}} + p}} < {p^t}. $ |
Theorem 1.1 Let Cpn be the cyclic group of order pn (n≥1) generated by σ. For any integer m≥1, NmK2(
the generators of order pl (1≤l≤n-1) are 〈b(σ-1)pk-1
For all the above symbols, b∈B and β=(β1, …, βm)∈
Proof Suppose we get a generating set of K2(
We follow the notations in Ref. [13]. Let
$ \Delta ' = \left\{ {\alpha = \left( {{\alpha _1}, \cdots ,{\alpha _{m + 1}}} \right) \in \mathbb{N}_ + ^{m + 1}\left| {{\alpha _1} \geqslant {p^n}} \right.} \right\}, $ |
$ \Lambda ' = \left\{ {\left( {\alpha ,i} \right) \in \mathbb{N}_ + ^{m + 1} \times \left\{ {1,2, \cdots ,m + 1} \right\}} \right\}. $ |
For (α, i)∈Λ′, let [α, i]=min{k∈
Then by Corollary 2.7 in Ref. [13] and the above discussion, NmK2(
generators: 〈btα-εi, ti〉, where b∈B, (α, i)∈Λ′>10;
relations: pw(α, i)〈btα-εi, ti〉=0, where w(α, i)=「logp[α, i].
It is sufficient to determine the set Λ′>10.
If α1=pn and at least one of αj with p
If α1=pk for some 1≤k < pn-1 and j is the smallest number such that p
If p
So one gets
$ \begin{array}{*{20}{c}} {\Lambda {'}_{ > 1}^0 = \left\{ {\left( {\alpha ,1} \right)\left| {{\alpha _1} = {p^n},\gcd } \right.\left( {p,{\alpha _2}, \cdots ,{\alpha _{m + 1}}} \right) = 1} \right\}}\\ { \cup \left\{ {\left( {\alpha ,i} \right)\left| {\begin{array}{*{20}{c}} {{\alpha _1} = pk,1 \le k < {p^{n - 1}},}\\ {\gcd \left( {p,{\alpha _2}, \cdots ,{\alpha _{m + 1}}} \right) = 1,}\\ {i \ne \min \left\{ {j\left| {{\alpha _j} ≢ 0\bmod p} \right.} \right\}} \end{array}} \right.} \right\}}\\ { \cup \left( {\mathop \cup \limits_{j = 2}^{m + 1} \left\{ {\left( {\alpha ,j} \right)\left| {\begin{array}{*{20}{c}} {{\alpha _1} = pk - h,}\\ {1 \le k \le {p^{n - 1}},}\\ {1 \le h < \rho } \end{array}} \right.} \right\}} \right).} \end{array} $ |
Let b∈B. For any β∈
·〈b(σ-1)pk-1xβ, (σ-1)〉 where 1≤k≤pn-1 and gcd (p, β1, …, βm)=1;
·〈b(σ-1)pkxβ-εi, xi〉 where 1≤k < pn-1, gcd (p, β1, …, βm)=1 and i≠min{j|βj
·〈b(σ-1)pk-hxβ-εi, xi〉 where 1≤k≤pn-1, 1≤h < p, 1≤i≤m.
The relations are
$ \cdot \;\;{p^{\left\lceil {{{\log }_p}\frac{{{p^n} + 1}}{{pk}}} \right\rceil }}\left\langle {b{{\left( {\sigma - 1} \right)}^{pk - 1}}{x^\beta },\left( {\sigma - 1} \right)} \right\rangle = 0, $ |
$ \cdot \;\;{p^{\left\lceil {{{\log }_p}\frac{{{p^n}}}{{pk}}} \right\rceil }}\left\langle {b{{\left( {\sigma - 1} \right)}^{pk}}{x^{\beta - {\varepsilon ^i}}},{x_i}} \right\rangle = 0, $ |
$ \cdot \;\;{p^{\left\lceil {{{\log }_p}\frac{{{p^n}}}{{pk - h}}} \right\rceil }}\left\langle {b{{\left( {\sigma - 1} \right)}^{pk - h}}{x^{\beta - {\varepsilon ^i}}},{x_i}} \right\rangle = 0. $ |
Then by Lemma 1.2, the result follows.
2 ExamplesExample 2.1 Let C4 be the cyclic group of order 4 generated by σ. Then NK2(
$ \left\{ {\left\langle {\left( {\sigma - 1} \right){x^{i - 1}},x} \right\rangle \left| {i \ge 1} \right.} \right\}, $ |
$ \left\{ {\left\langle {\left( {\sigma - 1} \right){x^{2i - 1}},\left( {\sigma - 1} \right)} \right\rangle \left| {i \ge 1} \right.} \right\}, $ |
and the generators of order 2 are
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^3}{x^{i - 1}},x} \right\rangle \left| {i \ge 1} \right.} \right\}, $ |
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^3}{x^{2i - 1}},\left( {\sigma - 1} \right)} \right\rangle \left| {i \ge 1} \right.} \right\}. $ |
$ {N^2}{K_2}\left( {{\mathbb{F}_2}\left[ {{C_4}} \right]} \right) \cong { \oplus _\infty }\left( {\mathbb{Z}/2\mathbb{Z}\; \oplus \;\mathbb{Z}/4\mathbb{Z}} \right) $ |
N2K2(
$ \left\{ {\left\langle {\left( {\sigma - 1} \right){x^{i - 1}}{y^j},x} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {\left( {\sigma - 1} \right){x^i}{y^{j - 1}},y} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {\left( {\sigma - 1} \right){x^{2i - 1}}{y^j},\left( {\sigma - 1} \right)} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {\left( {\sigma - 1} \right){x^{2i}}{y^{2j - 1}},\left( {\sigma - 1} \right)} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
and the generators of order 2 are
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^3}{x^{i - 1}}{y^j},x} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^3}{x^i}{y^{j - 1}},y} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^2}{x^{2i - 1}}{y^{2j - 1}},x} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^2}{x^{2i - 1}}{y^{j - 1}},y} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^3}{x^{2i - 1}}{y^j},\left( {\sigma - 1} \right)} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
$ \left\{ {\left\langle {{{\left( {\sigma - 1} \right)}^3}{x^{2i}}{y^{2j - 1}},\left( {\sigma - 1} \right)} \right\rangle \left| {i \ge 1} \right.,j \ge 1} \right\}, $ |
where x, y are indeterminates.
Corollary 2.1 NK1(
Proof Assume Cp2 is generated by σ. There is a Milnor square,
where ζp2 is a primitive p2-th root of unity and
$ \begin{array}{*{20}{c}} {N{K_2}\left( {\mathbb{Z}\left[ {{C_{{p^2}}}} \right]} \right) \to N{K_2}\left( {\mathbb{Z}\left[ {{\zeta _{{p^2}}}} \right]} \right) \oplus } \\ {N{K_2}\left( {\mathbb{Z}\left[ {{C_p}} \right]} \right) \to N{K_2}\left( {{\mathbb{F}_p}\left[ {{C_p}} \right]} \right) \to } \\ {N{K_1}\left( {\mathbb{Z}\left[ {{C_{{p^2}}}} \right]} \right) \to N{K_1}\left( {\mathbb{Z}\left[ {{\zeta _{{p^2}}}} \right]} \right) \oplus N{K_1}\left( {\mathbb{Z}\left[ {{C_p}} \right]} \right).} \end{array} $ |
Since
$ \begin{array}{*{20}{c}} {N{K_2}\left( {\mathbb{Z}\left[ {{C_p}} \right]} \right) \to N{K_2}\left( {{\mathbb{F}_p}\left[ {{C_p}} \right]} \right) \to } \\ {N{K_1}\left( {\mathbb{Z}\left[ {{C_{{p^2}}}} \right]} \right) \to 0.} \end{array} $ |
Moreover, we have NK2(
[1] |
Bass H. Algebraic K-theory[M]. New York: Benjamin Inc, 1968.
|
[2] |
Grothendieck A. Esquisse d'un programme pour une theorie des intersections sur les schemas generaux[C]//Berthelot P. Théorie des Intersections et Théoréme de Riemann-Roch, Berlin: Springer-Verlag, 1971: 1-77.
|
[3] |
Milnor J. Introduction to algebraic K-theory[M]. New Jersey: Princeton University Press, 1971.
|
[4] |
Quillen D. Higher algebraic K-theory I[C]//Bass H. Higher K-theories. Berlin: Springer-Verlag, 1973: 85-147.
|
[5] |
Farrell F T. The nonfiniteness of Nil[J]. Proceedings of the American Mathematical Society, 1977, 65(2): 215-216. DOI:10.1090/S0002-9939-1977-0450328-1 |
[6] |
Prasolov A V. Infiniteness of the group Nil[J]. Matematicheskie Zametki, 1982, 32(1): 9-12. |
[7] |
Vorst T. A survey on the K-theory of polynomial extensions[C]//Bak A. Algebraic K-theory, number theory, geometry and analysis. Berlin: Springer-Verlag, 1984: 422-441.
|
[8] |
Weibel C A. Mayer-Vietoris sequences and module structures on NK*[C]//Friedlander E M, Stein M R. Algebraic K-theory Evanston 1980. Berlin: Springer-Verlag, 1981: 466-493.
|
[9] |
Connolly F X, Silva M. The groups NrK0( |
[10] |
Weibel C A. NK0 and NK1 of the groups C4 and D4[J]. Commentarii Mathematici Helvetici, 2009, 84(2): 339-349. |
[11] |
Morris R A. Derivations of Witt vectors with application to K2 of truncated polynomial rings and Laurent series[J]. Journal of Pure and Applied Algebra, 1980, 18(1): 91-96. DOI:10.1016/0022-4049(80)90120-6 |
[12] |
Juan-Pineda D. On higher nil groups of group rings[J]. Homology, Homotopy and Applications, 2007, 9(2): 95-100. DOI:10.4310/HHA.2007.v9.n2.a3 |
[13] |
Van der Kallen W, Stienstra J. The relative K2 of truncated polynomial rings[J]. Journal of Pure and Applied Algebra, 1984, 34(2-3): 277-289. DOI:10.1016/0022-4049(84)90041-0 |
[14] |
Harmon D R. NK1 of finite groups[J]. Proceedings of the American Mathematical Society, 1987, 100(2): 229-232. |
[15] |
Guin-Waléry D, Loday J-L. Obstruction a l'excision en K-theorie algebrique[C]//Friedlander E M, Stein M R. Algebraic K-theory Evanston 1980. Berlin: Springer-Verlag, 1981: 179-216.
|