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

1,1

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 A371811 A162409

Adjacent sequences: A085644 A085645 A085646 * A085648 A085649 A085650

Keyword

base,easy,nonn

Author

Amarnath Murthy, Jason Earls, Jul 11 2003

Status

approved