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A098450
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Largest n-digit semiprime.
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6
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9, 95, 998, 9998, 99998, 999997, 9999998, 99999997, 999999991, 9999999997, 99999999997, 999999999997, 9999999999989, 99999999999997, 999999999999998, 9999999999999994, 99999999999999989, 999999999999999993, 9999999999999999991, 99999999999999999983
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OFFSET
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1,1
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LINKS
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FORMULA
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MATHEMATICA
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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, 18] (* Robert G. Wilson v, Dec 18 2012 *)
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PROG
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(PARI) apply( A098450(n)={n=10^n; until(bigomega(n-=1)==2, ); n}, [1..20]) \\ M. F. Hasler, Jan 01 2021
(Python)
from sympy import factorint
def semiprime(n): f = factorint(n); return sum(f[p] for p in f) == 2
def a(n):
an = 10**n - 1
while not semiprime(an): an -= 1
return an
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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STATUS
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approved
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