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!)
A085647 Semiprimes whose prime factors p*q have an equal number of decimal digits and p is not equal to q. 2
6, 10, 14, 15, 21, 35, 143, 187, 209, 221, 247, 253, 299, 319, 323, 341, 377, 391, 403, 407, 437, 451, 473, 481, 493, 517, 527, 533, 551, 559, 583, 589, 611, 629, 649, 667, 671, 689, 697, 703, 713, 731, 737, 767, 779, 781, 793, 799, 803, 817, 851, 869, 871 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Chai Wah Wu, Table of n, a(n) for n = 1..10369

MATHEMATICA

spddQ[n_]:=Module[{fi=FactorInteger[n][[All, 1]]}, PrimeOmega[n]==2 && fi[[2]]- fi[[1]]>0&&IntegerLength[fi[[1]]]==IntegerLength[fi[[2]]]]; Select[Range[900], spddQ]//Quiet (* Harvey P. Dale, Jul 14 2021 *)

PROG

(Python)

from sympy import sieve

A085647 = []

for n in range(3):

....pr = list(sieve.primerange(10**n, 10**(n+1)))

....for i, p in enumerate(pr, start=1):

........for q in pr[i:]:

............A085647.append(p*q)

A085647 = sorted(A085647)

# Chai Wah Wu, Aug 26 2014

CROSSREFS

Cf. A001358, A078972.

Sequence in context: A225706 A063078 A064452 * A072901 A162409 A226494

Adjacent sequences: A085644 A085645 A085646 * A085648 A085649 A085650

KEYWORD

base,easy,nonn

AUTHOR

Amarnath Murthy, Jason Earls, Jul 11 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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 2 05:15 EST 2023. Contains 359997 sequences. (Running on oeis4.)