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A124786 Numbers n such that n, sopfr(n) and sopfr(sopfr(n)) are all semiprimes. 1
4, 14, 33, 38, 46, 49, 62, 69, 94, 129, 133, 134, 166, 169, 177, 205, 213, 217, 254, 262, 309, 334, 361, 393, 422, 445, 469, 489, 493, 502, 505, 526, 529, 614, 669, 718, 753, 793, 817, 865, 886, 889, 913, 933, 934, 961, 974, 982, 993, 1006, 1077, 1126, 1142 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Cf. A001358 semiprimes, A001414 sopfr, A115585 Semiprimes with a semiprime sum of factors, A124785(n)=sopfr(A115585(n)).

LINKS

Table of n, a(n) for n=1..53.

EXAMPLE

Full table of {n, sopfr(n), sopfr(sopfr(n))}:

{4, 4, 4}, {14, 9, 6}, {33, 14, 9}, {38, 21, 10}, {46, 25, 10}, {62, 33, 14}, {69, 26, 15}, {94, 49, 14}, {129, 46, 25}, {134, 69, 26}, {166, 85, 22}, {177, 62, 33}, {213, 74, 39}, {217, 38, 21}, {254, 129, 46}, {262, 133, 26}, {309, 106, 55}, {334, 169, 26}, {393, 134, 69}, {422, 213, 74}, {445, 94, 49}, {489, 166, 85}, {502, 253, 34}, {526, 265, 58}.

MAPLE

isA001358 := proc(n) if numtheory[bigomega](n) = 2 then true ; else false ; fi ; end: A001414 := proc(n) local ifs; if n = 1 then 0; else ifs := ifactors(n)[2] ; add( op(1, i)*op(2, i), i=ifs) ; fi ; end: A081758 := proc(n) A001414(A001414(n)) ; end: isA124786 := proc(n) if isA001358(n) and isA001358(A001414(n)) and isA001358(A081758(n)) then true ; else false ; fi ; end: for n from 2 to 2000 do if isA124786(n) then printf("%d, ", n) ; fi : od: # R. J. Mathar, Sep 23 2007

CROSSREFS

Cf. A001358, A001414, A115585, A124785.

Sequence in context: A324042 A099590 A078432 * A322009 A228272 A011935

Adjacent sequences:  A124783 A124784 A124785 * A124787 A124788 A124789

KEYWORD

nonn

AUTHOR

Zak Seidov, Nov 07 2006

EXTENSIONS

Corrected and extended by R. J. Mathar, Sep 23 2007

STATUS

approved

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Last modified January 23 16:42 EST 2020. Contains 331172 sequences. (Running on oeis4.)