What it means
ではあるまいか expresses a tentative but often persuasive inference, roughly asking whether something is not the case while suggesting that it probably is. It is used to invite agreement, raise a rhetorical question, or present a conclusion indirectly rather than state it assertively. The form is especially common in formal writing, speeches, commentary, and serious discussion. With verbs and i-adjectives, の is normally inserted before ではあるまいか.
Usage note
ではあるまいか is formal and somewhat literary rather than casual everyday speech. The more neutral familiar counterpart is ではないだろうか, while ではないでしょうか is the polite counterpart; these forms should not be treated as simple requests for information in every context, because they often imply that the speaker expects agreement.
How to use it
この方法は有効ではないでしょうか。
Could this method be effective?
この判断は正しいのではないでしょうか。
Could this judgment be correct?
この制度は社会を変えるのではないでしょうか。
Could this system change society?
彼は専門家ではないでしょうか。
Could he be an expert?
この方法は有効ではあるまいか。
Could this method be effective?
この判断は正しいのではあるまいか。
Could this judgment be correct?
この制度は社会を変えるのではあるまいか。
Could this system change society?
彼は専門家ではあるまいか。
Could he be an expert?