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A122498 Padovan numbers that are semiprimes. 4
4, 9, 21, 49, 65, 86, 265, 1081, 1897, 2513, 7739, 97229, 128801, 299426, 922111, 1221537, 2839729, 62608681, 338356945, 53406819691, 2066337330754, 6363483400447, 8429820731201, 432062194544201, 7190854504969591, 12619069972000553, 16716708595637087 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The smallest candidate for the next term in the b-file is A000931(1958), which is composite with 239 digits and an unknown number of prime factors. - Hugo Pfoertner, Sep 07 2017

LINKS

Hugo Pfoertner, Table of n, a(n) for n = 1..54

Eric Weisstein's World of Mathematics, Semiprime

MAPLE

select(x-> numtheory[bigomega](x)=2, [(<<0|1|0>,

   <0|0|1>, <1|1|0>>^i)[1$2]$i=0..300])[];  # Alois P. Heinz, Aug 31 2017

MATHEMATICA

SemiprimeQ[1] := False SemiprimeQ[n_Integer] := Plus @@ (Last /@ FactorInteger[n]) == 2 a = Table[ SeriesCoefficient[ Series[x/(1 - x^2 - x^3), {x, 0, 50}], n], {n, 0, 50}] f[n_] = If[SemiprimeQ[a[[n]]] == True, a[[n]], {}] Flatten[Table[f[n], {n, 1, Length[a]}]]

CROSSREFS

Cf. A000931, A001358, A053409, A100891.

Sequence in context: A027973 A103040 A084861 * A132750 A297296 A048582

Adjacent sequences:  A122495 A122496 A122497 * A122499 A122500 A122501

KEYWORD

nonn

AUTHOR

Roger L. Bagula, Sep 15 2006

EXTENSIONS

More terms from Alois P. Heinz, Aug 31 2017

STATUS

approved

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Last modified October 18 14:52 EDT 2019. Contains 328161 sequences. (Running on oeis4.)