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A046388 Odd numbers of the form p*q where p and q are distinct primes. 41
15, 21, 33, 35, 39, 51, 55, 57, 65, 69, 77, 85, 87, 91, 93, 95, 111, 115, 119, 123, 129, 133, 141, 143, 145, 155, 159, 161, 177, 183, 185, 187, 201, 203, 205, 209, 213, 215, 217, 219, 221, 235, 237, 247, 249, 253, 259, 265, 267, 287, 291, 295, 299, 301, 303 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

These are the odd squarefree semiprimes.

These numbers n have the property that n is a Fermat pseudoprime for at least two bases 1 < b < n - 1. That is, b^(n - 1) = 1 (mod n). See sequence A175101 for a count of the number of bases. - Karsten Meyer, Dec 02 2010

The new Mathematica function returns the next element of this sequence after integer n. This program is more efficient than the original one that was based on sorting and selecting. - Lei Zhou, Dec 11 2012

LINKS

T. D. Noe, Table of n, a(n) for n=1..1000

MATHEMATICA

max = 300; A046388 = Sort@Flatten@Table[Prime[m] Prime[n], {n, 3, Ceiling[PrimePi[max/3]]}, {m, 2, n - 1}]; Select[A046388, # < max &] (* Alonso del Arte based on Robert G. Wilson v's program for A006881, Oct 24 2011 *)

%t f[n_]:=Block[{p1=Prime[Range[2, PrimePi[Max[3, NextPrime[Ceiling@Sqrt[n + 1] - 1]]]]], p2}, p2 = Table[Max[NextPrime[p1[[i]]], NextPrime[Ceiling[(n + 1)/p1[[i]]] - 1]], {i, Length[p1]}]; Min[p1*p2]]; NestList[f, 15, 50] (* Lei Zhou, Dec 11 2012 *)

PROG

(Haskell)

a046388 n = a046388_list !! (n-1)

a046388_list = filter ((== 2) . a001221) a056911_list

-- Reinhard Zumkeller, Jan 02 2014

(PARI) isok(n) = (n % 2) && (bigomega(n) == 2) && (omega(n)==2); \\ Michel Marcus, Feb 05 2015

CROSSREFS

Cf. A001358, A046315, A046404.

Different from A056913, A098905, A225375.

Cf. A001221, A056911, intersection of A005117 and A046315.

Sequence in context: A225375 A146166 A024556 * A056913 A002557 A128907

Adjacent sequences:  A046385 A046386 A046387 * A046389 A046390 A046391

KEYWORD

nonn

AUTHOR

Patrick De Geest, Jun 15 1998

EXTENSIONS

I removed some ambiguity in the definition and edited the entry, merging in some material from A146166. - N. J. A. Sloane, May 09 2013

STATUS

approved

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Last modified July 23 15:58 EDT 2019. Contains 325258 sequences. (Running on oeis4.)