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Old 2020-08-15, 11:21   #1
Aug 2020

11410 Posts
Default PRP residue question

If a mersenne number has more than two prime factors, i.e. Mn = A * B * composite_factor , are the residues from running PRP on Mn/A and Mn/A/B the same? Asking this because I saw M8325613, where M8325613/4504206586679 and M8325613/4504206586679/121878456335252812193 gave the same residue 88A96FEB41BFB7FD.

Last fiddled with by Ensigm on 2020-08-15 at 11:22
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Old 2020-08-15, 12:22   #2
ATH's Avatar
Dec 2003

2·7·229 Posts

Yes, if 1 type 5 PRP test has been done and another factor is found then only 1 type 5 test is needed because it gives the same residue because type 5 test is doing a^(N*knownfactors-1) (mod N).

So if Mn=A*B*C and factor A is found first then N=B*C and knownfactors=A, so it is doing:
a^(B*C * A - 1 ) (mod B*C)

Later when factor B is found then N=C and knownfactors=A*B, so it is doing:
a^(C * A*B - 1) (mod C)
so the exponent is the same and because C is a factor of B*C it will give the same 64bit residue.

The full residues are not equal but if we call the first one R1 and second one R2 then:
R1 (mod C) = R2.

Last fiddled with by ATH on 2020-08-15 at 13:09
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