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A258882 Primitive weird numbers of the form 2^k*p*q with k > 0 and where p < q are odd primes. 19
70, 836, 7192, 7912, 9272, 10792, 17272, 73616, 83312, 113072, 519712, 539744, 555616, 682592, 786208, 1188256, 1229152, 1901728, 2081824, 2189024, 3963968, 4128448, 4145216, 4486208, 4559552, 4632896, 4960448, 5440192, 5568448, 6460864, 6621632, 7354304, 7470272, 8000704, 8134208 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The number of terms < 10^n: 0, 1, 2, 5, 9, 15, 35, 61, 114, 204, 380, 696, 1703, 3548, 6726, 13137, ....

If 2^k*p*q is a weird number, it is necessarily primitive, and 2^(k+1) < p < 2^(k+2)-2 < q < 2^(2k+1).

No odd weird numbers are known and any even weird number must have at least 3 distinct prime factors, since all numbers of the form 2^k*p^m are deficient or pseudoperfect or perfect (iff m = 1 and p = 2^(k+1)-1 is a Mersenne prime). Sequence A258333 lists the number of terms in this sequence for given k. - M. F. Hasler, Jul 11 2016

Kravitz has shown that 2^k*p*q is a primitive weird number when the primes p and q satisfy p = (2^(k+1)*q-q-1)/(q+1-2^(k+1)). Many terms in this sequence are of this form, e.g., a(n) with n = 1, 2, 3, 4, 6, 7, 9, 10, 15, 23, 26, 38, 45, 75, 94, 144, 157, 187, 287, 327, 368, 370, 459, 607, 657, 658, .... Sequences A242025, A242998, ... are related to the special case where q is a Mersenne prime (A000668). - M. F. Hasler, Jul 12 2016

Weird numbers of the form 2^k*p*q are always primitive, so this condition could be omitted in the definition of this sequence. In the finite subsequences of terms for given k (cf. A258333), it appears that the smaller factor p is decreasing and the larger factor q is increasing, as the terms grow. - M. F. Hasler, Jul 13 2016

REFERENCES

S. Kravitz, A search for large weird numbers. J. Recreational Math. 9 (1976), 82-85 (1977). Zbl 0365.10003

LINKS

Douglas E. Iannucci and Robert G. Wilson v, Table of n, a(n) for n = 1..15384, updated Dec 06 2015; corrected by M. F. Hasler, Jul 16 2016

Douglas E. Iannucci, On primitive weird numbers of the form 2^k*p*q, arXiv:1504.02761 [math.NT], 2015.

Linked In, Number Theory, A very big weird number

Giuseppe Melfi, On the conditional infiniteness of primitive weird numbers, Journal of Number Theory, Vol. 147, Feb 2015, pgs 508-514.

Eric Weisstein's World of Mathematics, Weird Number.

Wikipedia, Weird number

FORMULA

A258882 union A258401 is A002975.

EXAMPLE

a(1) = A002975(1) = 70 = 2*5*7.

a(2) = A002975(2) = 836 = 2^2*11*19.

A002975(3) = 4030 = 2*5*13*31 is not in this sequence since it is not of the required form.

The same is true for A002975(4) = 5830.

a(3) = A002975(5) = 7192 = 2^3*29*31, etc.

MATHEMATICA

(* copy the terms from A002975, assign them equal to 'lst' and then *) fQ[n_] := Block[{m = n}, While[ Mod[m, 2] == 0, m /= 2]; PrimeOmega@ m == 2]; Select[lst, fQ]

PROG

(PARI) select(t->factor(t)[, 2][^1]=[1, 1]~, A002975) \\ Assuming that A002975 is defined as set or vector. - M. F. Hasler, Jul 11 2016

CROSSREFS

Cf. A002975, A258401, A258374, A258375, A258883, A258884, A258885.

Cf. A242025, A242993, A242998, A242999, A243003 (related to the subsequence with q = (2^k*p-p-1)/(p+1-2^k) and p a Mersenne prime).

Sequence in context: A006037 A002975 A258250 * A265726 A258375 A251933

Adjacent sequences:  A258879 A258880 A258881 * A258883 A258884 A258885

KEYWORD

nonn

AUTHOR

Douglas E. Iannucci and Robert G. Wilson v, Jun 14 2015

EXTENSIONS

Edited by M. F. Hasler, Jul 11 2016

STATUS

approved

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Last modified February 24 11:39 EST 2018. Contains 299603 sequences. (Running on oeis4.)