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

Offset: 1

Author

Amarnath Murthy, Jason Earls, Jul 11 2003

Keywords

• base
• easy
• nonn

Links

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

Crossrefs

Cf. A001358, A078972.

Programs

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 *)

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