A098449 Smallest n-digit semiprime.

4, 10, 106, 1003, 10001, 100001, 1000001, 10000001, 100000001, 1000000006, 10000000003, 100000000007, 1000000000007, 10000000000015, 100000000000013, 1000000000000003, 10000000000000003, 100000000000000015, 1000000000000000007, 10000000000000000001

Offset

1,1

Links

Derek Orr, Table of n, a(n) for n = 1..60

Mathematica

NextSemiPrime[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sp = n + sgn; While[c < Abs[k], While[ PrimeOmega[sp] != 2, If[sgn < 0, sp--, sp++]]; If[sgn < 0, sp--, sp++]; c++]; sp + If[sgn < 0, 1, -1]]; f[n_] := NextSemiPrime[10^n - 1]; Array[f, 19, 0] (* Robert G. Wilson v, Dec 18 2012 *)

Prog

(PARI)
a(n)=for(k=10^(n-1), 10^n-1, if(bigomega(k)==2, return(k)))
vector(50, n, a(n)) \\ Derek Orr, Aug 15 2014
(Python)
from sympy import factorint
def semiprime(n): f = factorint(n); return sum(f[p] for p in f) == 2
def a(n):
  an = max(1, 10**(n-1))
  while not semiprime(an): an += 1
  return an
print([a(n) for n in range(1, 21)]) # Michael S. Branicky, Apr 10 2021

Crossrefs

Cf. A098450 (largest n-digit semiprime), A003617 (smallest n-digit prime), A001358 (semiprimes).

Sequence in context: A197851 A197865 A267128 * A118378 A203224 A198156

Adjacent sequences: A098446 A098447 A098448 * A098450 A098451 A098452

Keyword

base,nonn

Author

Rick L. Shepherd, Sep 07 2004

Status

approved