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Old 2019-03-14, 18:35   #23
paulunderwood
 
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Sep 2002
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Quote:
Originally Posted by enzocreti View Post
Simply: number formed by the concatenation in base 10 of two consecutive Mersenne numbers.
Examples are 31, 73, 157, 3115, 40952047...
This will get you going:

Code:
for(a=1,100,A=2^a-1;VA=digits(A);for(b=1,100,B=2^b-1;VB=digits(b);VZ=concat(VA,VB);summand=0;pow=1;forstep(z=#VZ,1,-1,summand=summand+VZ[z]*pow;pow=pow*10);if(ispseudoprime(summand),print([summand]))))
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