http://oaspa.org/why-cc-by/

How to Use GNU Licenses for Your Own Software	http://www.gnu.org/copyleft/gpl-howto.html
FDL - GNU Free Documentation License	http://www.gnu.org/licenses/fdl.html
Tips on Using the GNU Free Documentation License	http://www.gnu.org/copyleft/fdl-howto.html


Copyleft	http://www.gnu.org/copyleft/

Copyleft Symbol
The copyleft symbol is a backwards C in a circle (copyright symbol © mirrored). It has no legal significance.</p>
<p>Because it is unavailable on Unicode, it can be approximated with character U+2184 ↄ LATIN SMALL LETTER REVERSED C or the more widely available character U+0254 ɔ LATIN SMALL LETTER OPEN O between parenthesis '(ɔ)' or, if supported by the application, by combining it with the character U+20DD ↄ⃝ COMBINING ENCLOSING CIRCLE: 'ↄ⃝'.[46] A discussion on the Unicode mailing list in July 2012, contended that there are other ways to insert the copyleft symbol, so it need not be encoded.</p>

https://en.wikipedia.org/wiki/Copyleft#Symbol