Search for other journals using this library database:
arXiv is a free distribution service and an open-access archive for 2,110,790 scholarly articles in the fields of physics, mathematics, computer science, quantitative biology, quantitative finance, statistics, electrical engineering and systems science, and economics.
From Cornell University, Historical Mathematics Monographs is a collection of selected monographs with expired copyrights chosen from the mathematics field.
Online forum for questions and answers on math topics
MathWorld is an online mathematics reference work, it includes definitions and links to resources for all levels of mathematics.
zbMATH Open (formerly known as Zentralblatt MATH) is the world's most comprehensive and longest-running abstracting and reviewing service in pure and applied mathematics.
Background sources are found in books and online sources. They provide general information about your topic such as an overview, definitions, key terms or persons, and suggestions for more sources. Sources are often books such as dictionaries, encyclopedias, handbooks, although they may be websites, articles, and similar tools. A database with some good background sources is CREDO Reference.
CREDO is listed in the Database list. 
Examples of background reference sources in Boreham:
