login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo

Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS").
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A088986 Numbers n such that each of n through n+4 are divisible by exactly two primes. 1
54, 91, 92, 115, 141, 142, 143, 144, 158, 205, 212, 213, 214, 215, 295, 301, 323, 324, 325, 391, 535, 685, 721, 799, 1135, 1345, 1465, 1535, 1711, 1941, 1981, 2101, 2215, 2302, 2303, 2304, 2425, 2641, 3865, 4411, 5461, 6505, 6625, 6925, 7165, 7231, 7261 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Identical with A045933 from first-to 38th terms, but deviates later because A045933 includes start of chains with more than 2 prime-factors.

Contrary to longer chains(6,7,8,..) of omega=2 this sequence seems to be either infinite or very long. See A088963-A088985.

Primes counted without multiplicity. [Harvey P. Dale, Oct 20 2011]

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..750

EXAMPLE

n=9997771+j for j=0,1,2,3,4 has 2 distinct prime-factors:

{7.1428253 # 4.2499443 # 3.3332591 # 2.4998887 # 25.399911}.

MATHEMATICA

Transpose[Select[Partition[Transpose[Select[Table[{n, PrimeNu[n]}, {n, 10000}], Last[#]==2&]][[1]], 5, 1], Last[#]-First[#]==4&]][[1]] (* Harvey P. Dale, Oct 20 2011 *)

PROG

(Python)

from sympy import primefactors

def ok(n):

for i in range(5):

        if len(primefactors(n + i))!=2: return 0

    return 1

print [n for n in range(1, 8001) if ok(n)] # Indranil Ghosh, Jul 17 2017

CROSSREFS

Sequence in context: A290146 A071863 A045933 * A259717 A118150 A039779

Adjacent sequences:  A088983 A088984 A088985 * A088987 A088988 A088989

KEYWORD

nonn

AUTHOR

Labos Elemer, Sep 30 2003

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 25 20:53 EST 2020. Contains 338627 sequences. (Running on oeis4.)