The Double-Struck N-Ary Summation symbol (⅀). Copy and paste it, or use its Unicode, HTML, and CSS codes.
More from Letterlike Symbols.
The quickest way is to copy ⅀ from this page and paste it anywhere. You can also insert it with its Unicode, HTML, or alt code listed above.
⅀ is U+2140 (decimal 8512).
Use ⅀ in HTML to render ⅀ on a web page.
On Windows you can type it with Alt+8512 (hold Alt and enter the number on the numeric keypad).
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