

A091155


Numbers n such that n  2^k is squarefree for all 1 <= 2^k < n.


1



2, 3, 4, 7, 15, 23, 39, 63, 75, 87, 111, 135, 147, 159, 195, 219, 231, 255, 267, 315, 387, 399, 411, 423, 435, 447, 459, 495, 519, 567, 615, 663, 675, 699, 711, 735, 747, 759, 771, 819, 867, 915, 999, 1011, 1023, 1035, 1047, 1071, 1095, 1119, 1155, 1167, 1263
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OFFSET

1,1


COMMENTS

Erdos conjectures that this sequence is infinite. It appears that n = 3 (mod 12) except for n = 2, 4, 7 and 23.


REFERENCES

R. K. Guy, Unsolved Problems in Number Theory, A19.


LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000
P. Erdõs, On integers of the form 2^k + p and some related problems, Summa Brasil. Math. 2 (1950), pp. 113123.


EXAMPLE

39 is on the list because 38, 37, 35, 31, 23 and 7 are all squarefree.


MATHEMATICA

a={}; Do[k=1; While[sf=SquareFreeQ[nk]; sf&&2k<n, k=2k]; If[sf, AppendTo[a, n]], {n, 2000}]; a


PROG

(PARI) is(n)=for(k=1, log(n+.5)\log(2), if(!issquarefree(n2^k), return(0))); 1 \\ Charles R Greathouse IV, Apr 13 2014


CROSSREFS

Cf. A039669 (n such that n2^k are all primes).
Sequence in context: A265742 A098010 A088533 * A254432 A027362 A068194
Adjacent sequences: A091152 A091153 A091154 * A091156 A091157 A091158


KEYWORD

nonn


AUTHOR

T. D. Noe, Dec 23 2003


STATUS

approved



