## Journal of the Mathematical Society of Japan

- J. Math. Soc. Japan
- Volume 53, Number 4 (2001), 975-990.

### Gap modules for direct product groups

#### Abstract

Let $G$ be a finite group. A gap $G$-module $V$ is a finite dimensional real $G$-representation space satisfying the following two conditions:

(1) The following strong gap condition holds: $\mathrm{dim}{V}^{P}>2\mathrm{dim}{V}^{H}$ for all $$$H\le G$ such that $P$ is of prime power order, which is a sufficient condition to define a Gsurgery obstruction group and a $G$-surgery obstruction.

(2)$V$ has only one $H$-fixed point 0 for all large subgroups $H$, namely $H\u220a\mathcal{L}\left(G\right)$. A finite group $G$ not of prime power order is called a gap group if there exists a gap Gmodule. We discuss the question when the direct product $K\times L$ is a gap group for two finite groups $K$ and $L$. According to [5], if $K$ and $K\times {C}_{2}$ are gap groups, so is $K\times L$. In this paper, we prove that if $K$ is a gap group, so is $K\times {C}_{2}$. Using [5], this allows us to show that if a finite group $G$ has a quotient group which is a gap group, then $G$ itself is a gap group. Also, we prove the converse: if $K$ is not a gap group, then $K\times {D}_{2n}$ is not a gap group. To show this we define a condition, called NGC, which is equivalent to the non-existence of gap modules.

#### Article information

**Source**

J. Math. Soc. Japan, Volume 53, Number 4 (2001), 975-990.

**Dates**

First available in Project Euclid: 29 May 2008

**Permanent link to this document**

https://projecteuclid.org/euclid.jmsj/1212067582

**Digital Object Identifier**

doi:10.2969/jmsj/05340975

**Mathematical Reviews number (MathSciNet)**

MR1852892

**Zentralblatt MATH identifier**

1065.20004

**Subjects**

Primary: 57S17: Finite transformation groups 20C15: Ordinary representations and characters

**Keywords**

gap group gap module real representation direct product

#### Citation

SUMI, Toshio. Gap modules for direct product groups. J. Math. Soc. Japan 53 (2001), no. 4, 975--990. doi:10.2969/jmsj/05340975. https://projecteuclid.org/euclid.jmsj/1212067582