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

 

Logo

Please make a donation to keep the OEIS running. We are now in our 55th year. In the past year we added 12000 new sequences and reached 8000 citations (which often say "discovered thanks to the OEIS"). We need to raise money to hire someone to manage submissions, which would reduce the load on our editors and speed up editing.
Other ways to donate

Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A276564 Perfect powers n (exponent greater than 1) such that n-1 and n+1 are both semiprime. 1
144, 216, 900, 1764, 2048, 3600, 10404, 11664, 39204, 97344, 213444, 248832, 272484, 360000, 656100, 685584, 1040400, 1102500, 1127844, 1633284, 2108304, 2214144, 3504384, 3802500, 4112784, 4536900, 4588164, 5475600, 7784100, 7851204, 8388608, 8820900, 9000000, 9734400 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Intersection of A001597 and A124936. - Michel Marcus, Dec 03 2016

LINKS

Table of n, a(n) for n=1..34.

EXAMPLE

2048 = 2^11, and both 2047 = 23*89 and 2049 = 3*683 are semiprime.

MATHEMATICA

Select[Range[10^7], And[GCD @@ FactorInteger[#][[All, 2]] > 1, Union@ # == {2} &@ Map[PrimeOmega, {# - 1, # + 1}]] &] (* Michael De Vlieger, Dec 07 2016, after Ant King at A001597 *)

PROG

(PARI) for(i=2, 10^7, if(ispower(i)&&bigomega(i-1)==2&&bigomega(i+1)==2, print1(i, ", ")))

CROSSREFS

Cf. A001597, A108278, A121898, A124936, A167023, A268043, A276565.

Sequence in context: A135780 A069701 A124144 * A061040 A159456 A316483

Adjacent sequences:  A276561 A276562 A276563 * A276565 A276566 A276567

KEYWORD

nonn

AUTHOR

Antonio Roldán, Nov 16 2016

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 December 8 02:30 EST 2019. Contains 329850 sequences. (Running on oeis4.)