A085647 - OEIS

A085647

Semiprimes whose prime factors p*q have an equal number of decimal digits and p is not equal to q.

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