login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A176652 Numbers k such that both semiprime(k)/p and semiprime(k+1)/p are prime for some prime p. 1
1, 2, 4, 6, 21, 42, 87, 120, 141, 142, 168, 179, 185, 188, 245, 255, 320, 363, 387, 434, 464, 496, 539, 593, 675, 697, 721, 753, 794, 810, 894, 929, 995, 1023, 1032, 1060, 1080, 1081, 1105, 1147, 1166, 1221, 1224, 1228, 1275, 1356, 1391, 1477, 1478, 1498 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

Indices n such that A001358(n) and A001358(n+1) share one prime factor. - R. J. Mathar, Apr 26 2010

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

2 is a term because both semiprime(2)/3 = 6/3 = 2 and semiprime(2+1)/3 = 9/3 = 3 are prime.

MAPLE

isA176652 := proc(n) pfsn := convert(numtheory[factorset]( A001358(n) ), list) ; pfsn1 := convert(numtheory[factorset]( A001358(n+1) ), list) ; op(1, pfsn) = op(1, pfsn1) or op(1, pfsn) = op(-1, pfsn1) or op(-1, pfsn) = op(1, pfsn1) or op(-1, pfsn) = op(-1, pfsn1) ; end proc: for n from 1 to 1600 do if isA176652(n) then printf("%d, ", n) ; end if; end do: # R. J. Mathar, Apr 26 2010

MATHEMATICA

sppQ[{a_, b_}]:=Module[{af=FactorInteger[a][[All, 1]], bf=FactorInteger[b][[All, 1]]}, Length[Intersection[af, bf]]==1]; Position[Partition[ Select[ Range[7000], PrimeOmega[#]==2&], 2, 1], _?sppQ]//Flatten (* Harvey P. Dale, Oct 08 2017 *)

CROSSREFS

Cf. A001358.

Sequence in context: A245766 A193774 A241210 * A251724 A326363 A273522

Adjacent sequences:  A176649 A176650 A176651 * A176653 A176654 A176655

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, Apr 22 2010

EXTENSIONS

Extended beyond 141 by R. J. Mathar, Apr 26 2010

Name clarified by Jon E. Schoenfield, Feb 06 2019

STATUS

approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified November 15 01:15 EST 2019. Contains 329142 sequences. (Running on oeis4.)