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

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A357075 Numbers sandwiched between numbers with exactly three distinct prime factors. 0
131, 139, 155, 169, 181, 221, 229, 239, 259, 265, 281, 307, 309, 311, 341, 349, 365, 371, 373, 379, 407, 409, 439, 441, 443, 469, 475, 491, 493, 505, 517, 519, 521, 529, 531, 533, 551, 559, 573, 581, 589, 599, 601, 611, 617, 619, 637, 643, 645, 664, 671, 679, 681, 683 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
Number k such that both k-1 and k+1 are in A033992.
LINKS
EXAMPLE
131 is sandwiched between 130 = 2*5*13 and 132 = 2^2*3*11. Both 130 and 132 have exactly three prime factors. Thus, 131 is in this sequence.
MATHEMATICA
Select[Range[1000], Length[FactorInteger[# + 1]] == 3 && Length[FactorInteger[# - 1]] == 3 &]
PROG
(Python)
from sympy import factorint
def isA033992(n): return len(factorint(n)) == 3
def ok(n): return isA033992(n-1) and isA033992(n+1)
print([k for k in range(700) if ok(k)]) # Michael S. Branicky, Sep 10 2022
(PARI) is(n)=omega(n-1)==3 && omega(n+1)==3 \\ Charles R Greathouse IV, Sep 11 2022
(PARI) list(lim)=my(v=List(), a=3, b, c); forfactored(n=132, lim\1+1, c=#n[2]~; if(c==3 && a==3, listput(v, n[1]-1)); a=b; b=c); Vec(v) \\ Charles R Greathouse IV, Sep 28 2022
CROSSREFS
Sequence in context: A210498 A050671 A095317 * A116519 A139646 A139970
KEYWORD
nonn,easy
AUTHOR
Tanya Khovanova, Sep 10 2022
STATUS
approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified March 28 04:05 EDT 2024. Contains 371235 sequences. (Running on oeis4.)