Comments on: Prime Riff http://www.josephzitt.com/wordpress/archives/24 Joseph Zitt on writing, selling, and considering books and music. Wed, 07 Jan 2009 04:16:25 +0000 http://wordpress.org/?v=2.7 hourly 1 By: joseph.zitt http://www.josephzitt.com/wordpress/archives/24/comment-page-1#comment-197 joseph.zitt Wed, 03 Jan 2007 11:44:01 +0000 http://www.josephzitt.com/wordpress/archives/24#comment-197 Ah, right, I forgot the "casting out nines." It's a useful tool, with which I'm rusty, and it's been a few decades since I'd thought of it. Ah, right, I forgot the “casting out nines.” It’s a useful tool, with which I’m rusty, and it’s been a few decades since I’d thought of it.

]]> By: msmas http://www.josephzitt.com/wordpress/archives/24/comment-page-1#comment-195 msmas Tue, 02 Jan 2007 14:59:01 +0000 http://www.josephzitt.com/wordpress/archives/24#comment-195 I liked the cash register part. I liked the cash register part.

]]> By: John Cowan http://www.josephzitt.com/wordpress/archives/24/comment-page-1#comment-194 John Cowan Tue, 02 Jan 2007 14:38:33 +0000 http://www.josephzitt.com/wordpress/archives/24#comment-194 You forgot the rule of "casting out nines": if the sum of the digits is divisible by nine (or three), so is the original number. 2+0+0+7=9, so 9 is a factor, but 2+2+3=7, so 9 is not a factor. There's a similar rule for divisibility by 11, where you attach alternating signs to the digits starting from the right-hand end; but it's not worth applying for small numbers, as multiples of 11 are usually obvious. You forgot the rule of “casting out nines”: if the sum of the digits is divisible by nine (or three), so is the original number. 2+0+0+7=9, so 9 is a factor, but 2+2+3=7, so 9 is not a factor.

There’s a similar rule for divisibility by 11, where you attach alternating signs to the digits starting from the right-hand end; but it’s not worth applying for small numbers, as multiples of 11 are usually obvious.

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