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A098146 First odd semiprime > 10^n. 1
9, 15, 111, 1003, 10001, 100001, 1000001, 10000001, 100000001, 1000000013, 10000000003, 100000000007, 1000000000007, 10000000000015, 100000000000013, 1000000000000003, 10000000000000003, 100000000000000015 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

LINKS

Table of n, a(n) for n=0..17.

Dario Alpern, Factorization using the Elliptic Curve Method.

EXAMPLE

a(0)=9 because 9=3*3 is the first odd semiprime following 10^0=1, a(13)=10000000000015=5*2000000000003.

MATHEMATICA

osp[n_]:=Module[{k=1}, While[PrimeOmega[n+k]!=2, k=k+2]; n+k]; Join[{9}, Table[osp[10^i], {i, 20}]] (* Harvey P. Dale, Jan 17 2012 *)

PROG

(PARI) print1(9, ", "); for(n=1, 10, forstep(i=10^n+1, 10^(n+1)-1, 2, f=factor(i); ms=matsize(f); if((ms[1]==1&&f[1, 2]==2)||(ms[1]==2&&f[1, 2]==1&&f[2, 2]==1), print1(i, ", "); break))) - Herman Jamke (hermanjamke(AT)fastmail.fm), Oct 21 2006

CROSSREFS

Cf. A046315 odd semiprimes, A098147(n)=a(n)-10^n continuation of this sequence, A003717 smallest n-digit prime.

Sequence in context: A136353 A136354 A177184 * A124274 A075134 A316744

Adjacent sequences:  A098143 A098144 A098145 * A098147 A098148 A098149

KEYWORD

nonn

AUTHOR

Hugo Pfoertner, Aug 28 2004

STATUS

approved

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Last modified July 29 12:41 EDT 2021. Contains 346346 sequences. (Running on oeis4.)