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Old 2010-11-12, 20:01   #42
science_man_88
 
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Jul 2009
Dumbassville

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for example 2047 = 23*89

2+4+7=4 mod 9
2+3+8+9 = 4 mod 9
5+8 = 4 mod 9

so the sumdigits for each multiplier 2kp+1 summed together gives the same as the sumdigits for 2^p+1.
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