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A182085 Carmichael numbers of the form (30k+7)*(60k+13)*(150k+31). 1
2821, 488881, 288120421, 492559141, 776176261, 1632785701, 3835537861, 6735266161, 9030158341, 21796387201, 167098039921, 288374745541, 351768558961, 381558955141, 505121232001, 582561482161, 915245066821, 2199733160881, 2402435763841, 4541477778181 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Note that in this sequence, 30k+7, 60k+13, and 150k+31 do not have to be prime. These numbers were found by taking the intersection of Carmichael numbers found by Pinch and numbers of the form (30k+7)*(60k+13)*(150k+31).

Conjecture: N = (30k+7)*(60k+13)*(150k+31) is a Carmichael number if (but not only if) 30k+7, 60k+13 and 150k+31 are all three prime numbers.

We checked the conjecture up to k = 256; we got Carmichael numbers with three prime divisors for k = 0, 1, 10, 12, 18, 24, 32, 43, 85, 102, 123, 129, 150, 201, 207, 256.

We got Carmichael numbers with more than three prime divisors for n = 14, 29, 109, 112.

All these numbers can be written as well as N = (n+1)*(2n+1)*(5n+1), where n = 30k+6.

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

PROG

(PARI) test(lim)={

    my(v=List(), n, f);

    for(k=0, lim,

        n=(30*k+7)*(60*k+13)*(150*k+31)-1;

        f=factor(30*k+7);

        for(i=1, #f[, 1], if(f[i, 2]>1 || n%(f[i, 1]-1), next(2)));

        f=factor(60*k+13);

        for(i=1, #f[, 1], if(f[i, 2]>1 || n%(f[i, 1]-1), next(2)));

        f=factor(150*k+31);

        for(i=1, #f[, 1], if(f[i, 2]>1 || n%(f[i, 1]-1), next(2)));

        listput(v, n+1)

    );

    Vec(v)

}; \\ Charles R Greathouse IV, Oct 02 2012

CROSSREFS

Cf. A002997 (Carmichael numbers), A087788.

Sequence in context: A248985 A236794 A182151 * A271580 A237063 A251014

Adjacent sequences:  A182082 A182083 A182084 * A182086 A182087 A182088

KEYWORD

nonn

AUTHOR

Marius Coman, Apr 11 2012

EXTENSIONS

Extended and corrected by T. D. Noe, Apr 19 2012

STATUS

approved

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Last modified October 31 00:25 EDT 2020. Contains 338095 sequences. (Running on oeis4.)