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Welcome! We are an encyclopedia project, and more. We are a different sort of Web 2.0 project: We aim at credibility and quality, not just quantity. Both the general public and credentialed experts are encouraged to get involved. We use our real names, not pseudonyms. We're collegial. We have added over 3,400 articles (and many subpages) since November 2006. Read our first year progress report and press release. Join us! New authors and editors welcome! A human being will respond usually within a few hours. Learn about us For readers: About the Citizendium For authors: The Author Role For editors: The Editor Role Initiatives November 7 to 13 only: Stub Week - an experiment that encourages short articles Ongoing: Core Articles - tackle high priority topics Recruitment - help invite others to join Subpages - add other types of info to CZ Eduzendium - students write CZ articles for credit Support us We're a 501(c)(3) non-profit, and we need your support. If you shop at Amazon.com or Barnes & Noble, do so through our links! We'll get donations at no cost to you. (read more) - Search All Products.	Some of our finest [ about ] Approved Articles Developed Articles (over 600) Article of the Week [ about ] A prime number is a number that can be evenly divided by exactly two positive whole numbers, namely 1 and itself. The first few prime numbers are 2, 3, 5, 7, 11, 13, and 17. A prime number $p$ cannot be factored as the product of two numbers $\scriptstyle p=a\times b,$ except for the trivial factorizations $\scriptstyle p=1\times p=p\times 1$ that all numbers possess. With the exception of 2, all prime numbers are odd numbers, but not every odd number is prime. For example, $\scriptstyle 9=3\times 3$ and $\scriptstyle 15=3\times 5,$ so neither 9 nor 15 is prime. By the strict mathematical definition, the number 1 is not considered to be prime (although this is a matter of definition, and mathematicians in the past often did consider 1 to be a prime). [more...] New Draft of the Week [ about ] The Red Sea Urchin, Strongylocentrotus franciscanus (A. Agassiz, 1863), is a species of marine invertebrate belonging to the phylum Echinodermata or "spiny-skinned" animals. Typically found in the Pacific ocean from Alaska to Baja California, red urchins inhabit shallow waters from the low-tide line to depths of 100 m. In general, this species prefers wave-sheltered rocky shorelines. [more...]