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A182208 Carmichael numbers divisible by 7. 1
1729, 2821, 6601, 8911, 15841, 41041, 52633, 63973, 101101, 126217, 172081, 188461, 670033, 748657, 825265, 838201, 997633, 1033669, 1082809, 1773289, 2628073, 4463641, 4909177, 6840001, 7995169, 8719921, 8830801, 9585541, 9890881 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Conjecture: Any Carmichael number C divisible by 7 can be written in one of two ways: (1) C=7*(6m+1)*(6n+1), where m and n are natural numbers or (2) C=7*(6m-1)*(6n-1), where m and n are natural numbers. In other words, there aren’t Carmichael numbers divisible by 7 of the form C=7*(6m+1)*(6n-1). Checked for the first 27 Carmichael numbers divisible by 7. Note: a Carmichael number with more than 3 prime divisors can be written (sometimes) in both ways: 41041 = 7*11*13*41 = 7*13*451 (form 1) = 7*11*533 = 7*41*143 (form 2).

Observation: in the first 100 Carmichael numbers with three prime divisors (not divisible by 3) there is no one to can be written as (6x+1)(6y+1)(6z-1), they are all of the form (6x+1)(6y+1)(6z+1), (6x-1)(6y-1)(6z-1) or (6x+1)(6y-1)(6z-1). Would not that be enough to make an assumption that there are no such Carmichael numbers with three prime divisors, or even more, that aren't Carmichael numbers even with more than three divisors to can be written this way?

The conjecture follows from Korselt's criterion. - Charles R Greathouse IV, Oct 02 2012

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

E. W. Weisstein, MathWorld: Carmichael Number

MATHEMATICA

CarmichaelNbrQ[n_] := ! PrimeQ[n] && Mod[n, CarmichaelLambda@ n] == 1; 7 Select[ Range[2500000], CarmichaelNbrQ[ 7#] &] (* Robert G. Wilson v, Aug 24 2012 *)

PROG

(PARI) Korselt(n)=my(f=factor(n)); for(i=1, #f[, 1], if(f[i, 2]>1||(n-1)%(f[i, 1]-1), return(0))); 1

forstep(n=49, 1e6, 42, if(Korselt(n), print1(n", "))) \\ Charles R Greathouse IV, Oct 02 2012

CROSSREFS

Intersection of A002997 (Carmichael) and A008589 (multiples of 7). - Michel Marcus, Oct 11 2016

Sequence in context: A198775 A154729 A083737 * A324316 A182207 A138129

Adjacent sequences:  A182205 A182206 A182207 * A182209 A182210 A182211

KEYWORD

nonn

AUTHOR

Marius Coman, Apr 18 2012

EXTENSIONS

Corrected by Robert G. Wilson v, Aug 24 2012

STATUS

approved

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Last modified August 24 03:51 EDT 2019. Contains 326260 sequences. (Running on oeis4.)