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!)
A115709 Pentagonal numbers (A000326) whose digit reversal is a semiprime (A001358). 5
12, 22, 51, 176, 330, 532, 590, 715, 782, 925, 1162, 1247, 1335, 1426, 1717, 3151, 3290, 4187, 5551, 7107, 7315, 7957, 10542, 10795, 11051, 11837, 12376, 14950, 15251, 15555, 15862, 16172, 17120, 19097, 19780, 20126, 22265, 24130, 24512, 26467, 26867, 30175 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
EXAMPLE
532 is the 19th pentagonal number and 235=5*47 is semiprime.
MATHEMATICA
Select[PolygonalNumber[5, Range[200]], PrimeOmega[IntegerReverse[#]]==2&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Jan 27 2019 *)
PROG
(Python)
from sympy import factorint
def ok(p): return sum(e for e in factorint(int(str(p)[::-1])).values()) == 2
print([p for p in (n*(3*n-1)//2 for n in range(143)) if ok(p)]) # Michael S. Branicky, Dec 22 2021
CROSSREFS
Sequence in context: A129639 A153361 A200197 * A115703 A014633 A298323
KEYWORD
nonn,base
AUTHOR
Giovanni Resta, Jan 31 2006
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 29 11:14 EDT 2024. Contains 371278 sequences. (Running on oeis4.)