The OEIS is supported by the many generous donors to the OEIS Foundation.

A174798
Numbers n such that 2*prime(n) and 2*prime(n+1) are consecutive semiprimes.
1
1, 3, 10, 33, 43, 49, 50, 57, 63, 100, 113, 120, 131, 140, 149, 159, 173, 195, 206, 224, 230, 277, 284, 303, 315, 320, 332, 366, 373, 394, 395, 401, 448, 463, 469, 471, 473, 477, 483, 484, 492, 513, 524, 530, 534, 537, 543, 555, 558, 576, 577, 592, 600, 608
OFFSET
1,2
There are no semiprimes strictly between 2*prime(n) and 2*prime(n+1).
A174956(A100484(a(n+1)))=A174956(A100484(a(n)))+1. [From Reinhard Zumkeller, Apr 03 2010]
FORMULA
A000040(a(n)) = A001358(A174797(n))/2.
EXAMPLE
a(1)=1 because 2*prime(1)=4=semiprime(1) and 2*prime(1+1)=6=semiprime(2).
a(2)=3 because 2*prime(3)=10=semiprime(4) and 2*prime(3+1)=14=semiprime(5).
MATHEMATICA
PrimePi[First[#]]&/@Select[Partition[Select[Range[20000], PrimeOmega[#] == 2&], 2, 1]/2, And@@IntegerQ/@#&] (* Harvey P. Dale, Nov 19 2011 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
EXTENSIONS
Edited, corrected and extended by Ray Chandler, Apr 07 2010
STATUS
approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified September 20 20:48 EDT 2024. Contains 376078 sequences. (Running on oeis4.)