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A070552 Numbers n such that n and n+1 are semiprimes. 28
9, 14, 21, 25, 33, 34, 38, 57, 85, 86, 93, 94, 118, 121, 122, 133, 141, 142, 145, 158, 177, 201, 202, 205, 213, 214, 217, 218, 253, 298, 301, 302, 326, 334, 361, 381, 393, 394, 445, 446, 453, 481, 501, 514, 526, 537, 542, 553, 565, 622, 633, 634, 694, 697 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

A064911(a(n))*A064911(a(n)+1) = 1. - Reinhard Zumkeller, Dec 03 2009

LINKS

D. W. Wilson, Table of n, a(n) for n = 1..10000

FORMULA

a(n) >> n log n since either n or n+1 is in A100484. - Charles R Greathouse IV, Jul 21 2015

MATHEMATICA

f[n_]:=Last/@FactorInteger[n]=={1, 1}||Last/@FactorInteger[n]=={2}; lst={}; Do[If[f[n]&&f[n+1], AppendTo[lst, n]], {n, 7!}]; lst (* Vladimir Joseph Stephan Orlovsky, Feb 25 2010 *)

Flatten[Position[Partition[Table[If[PrimeOmega[n]==2, 1, 0], {n, 700}], 2, 1], {1, 1}]] (* Harvey P. Dale, Feb 04 2015 *)

Select[Range[700], PrimeOmega[#] == PrimeOmega[# + 1] == 2 &] (* Vincenzo Librandi, Jan 22 2016 *)

PROG

(PARI) forprime(p=3, 1e3, if(bigomega(2*p-1)==2, print1(2*p-1", ")); if(bigomega(2*p+1)==2, print1(2*p", "))) \\ Charles R Greathouse IV, Nov 09 2011

(PARI) is(n)=if(n%2, isprime((n+1)/2) && bigomega(n)==2, isprime(n/2) && bigomega(n+1)==2) \\ Charles R Greathouse IV, Sep 08 2015

(MAGMA) IsSemiprime:=func< n | &+[k[2]: k in Factorization(n)] eq 2 >; [ n: n in [4..700] | IsSemiprime(n) and IsSemiprime(n+1) ]; // Vincenzo Librandi, Jan 22 2016

CROSSREFS

Cf. A001358, A007674, A039832, A100484.

Sequence in context: A272527 A272308 A186778 * A272141 A001198 A151915

Adjacent sequences:  A070549 A070550 A070551 * A070553 A070554 A070555

KEYWORD

nonn

AUTHOR

Sharon Sela (sharonsela(AT)hotmail.com), May 03 2002

EXTENSIONS

More terms from Vladeta Jovovic, May 03 2002

STATUS

approved

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Last modified June 29 02:06 EDT 2017. Contains 288858 sequences.