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A173703
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Composite numbers n with the property that phi(n) divides (n-1)^2.
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9
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561, 1105, 1729, 2465, 6601, 8481, 12801, 15841, 16705, 19345, 22321, 30889, 41041, 46657, 50881, 52633, 71905, 75361, 88561, 93961, 115921, 126673, 162401, 172081, 193249, 247105, 334153, 340561, 378561, 449065, 460801, 574561, 656601, 658801, 670033
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OFFSET
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1,1
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COMMENTS
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All terms are odd because if n is even, (n-1)^2 is odd and phi(n) is even for n > 2. - Donovan Johnson, Sep 08 2013
McNew showed that the number of terms in this sequence below x is O(x^(6/7)). - Tomohiro Yamada, Sep 28 2020
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LINKS
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EXAMPLE
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a(1) = 561 is in the sequence because 560^2 = phi(561)*980 = 320*980 = 313600.
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MAPLE
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isA173703 := proc(n)
n <> 1 and not isprime(n) and (modp( (n-1)^2, numtheory[phi](n)) = 0 );
end proc:
for n from 1 to 10000 do
if isA173703(n) then
printf("%d, \n", n);
end if;
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MATHEMATICA
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Union[Table[If[PrimeQ[n] === False && IntegerQ[(n-1)^2/EulerPhi[n]], n], {n, 3, 100000}]]
Select[Range[700000], CompositeQ[#]&&Divisible[(#-1)^2, EulerPhi[#]]&] (* Harvey P. Dale, Nov 29 2014 *)
Select[Range[1, 700000, 2], CompositeQ[#]&&PowerMod[#-1, 2, EulerPhi[ #]] == 0&] (* Harvey P. Dale, Aug 10 2021 *)
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PROG
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(PARI)
N=10^9;
default(primelimit, N);
ct = 0;
{ for (n=4, N,
if ( ! isprime(n),
if ( ( (n-1)^2 % eulerphi(n) ) == 0,
ct += 1;
print(ct, " ", n);
);
);
); }
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CROSSREFS
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Cf. A238574 (k-Lehmer numbers for some k).
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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