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 A237043 Numbers n such that 2^n - 1 is not squarefree, but 2^d - 1 is squarefree for every proper divisor d of n. 13
 6, 20, 21, 110, 136, 155, 253, 364, 602, 657, 812, 889, 979, 1081 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Primitive elements of A049094: the elements of A049094 are precisely the positive multiples of members of this sequence. If p^2 divides 2^n - 1 for some odd prime p, then by definition the multiplicative order of 2 mod p^2 divides n. The multiplicative order of 2 mod p^2 is p times the multiplicative order of 2 mod p unless p is a Wieferich prime, in which case the two orders are identical. Hence either p is a Wieferich prime or p*log_2(p+1) <= n. This should allow finding larger members of this sequence. - Charles R Greathouse IV, Feb 04 2014 If n is in the sequence and m>1 then m*n is not in the sequence. Because n is a proper divisor of m*n and 2^n-1 is not squarefree. - Farideh Firoozbakht, Feb 11 2014 a(15) >= 1207. - Max Alekseyev, Sep 28 2015 From Daniel Suteu, Jul 03 2019: (Start) The following numbers are also in the sequence: {1755, 2265, 2485, 2756, 3081, 3164, 4112, 6757, 8251, 13861, 18533}. Probably, the following numbers are also terms: {3422, 5253, 6806, 8164, 9316, 11342, 12550, 15025, 15026, 17030, 17404, 17468, 18145, 19670, 19701, 22052}. (End) LINKS Table of n, a(n) for n=1..14. MATHEMATICA Select[Range@ 160, And[AllTrue[2^#2 - 1, SquareFreeQ], ! SquareFreeQ[2^First@ #1 - 1]] & @@ TakeDrop[Divisors@ #, -1] &] (* Michael De Vlieger, Jul 07 2019 *) PROG (PARI) default(factor_add_primes, 1); isA049094(n)=my(f=factor(n>>valuation(n, 2))[, 1], N, o); for(i=1, #f, if(n%(f[i]-1) == 0, return(1))); N=2^n-1; fordiv(n, d, f=factor(2^d-1)[, 1]; for(i=1, #f, if(d==n, return(!issquarefree(N))); o=valuation(N, f[i]); if(o>1, return(1)); N/=f[i]^o)) is(n)=fordiv(n, d, if(isA049094(d), return(d==n))); 0 (PARI) \\ Simpler but slow is(n)=fordiv(n, d, if(!issquarefree(2^d-1), return(d==n))); 0 CROSSREFS Cf. A049094, A005420, A065069, A282631, A282632. Sequence in context: A222604 A112809 A289659 * A243905 A062017 A103678 Adjacent sequences: A237040 A237041 A237042 * A237044 A237045 A237046 KEYWORD nonn,hard,more AUTHOR Charles R Greathouse IV, Feb 02 2014 EXTENSIONS a(14) from Charles R Greathouse IV, Sep 21 2015, following Womack's factorization of 2^991-1. STATUS approved

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Last modified February 26 17:57 EST 2024. Contains 370352 sequences. (Running on oeis4.)