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A107913
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Numbers k that require three iterations of the sigma function to be >= 2*k.
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4
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9, 13, 25, 37, 43, 49, 61, 67, 73, 97, 109, 121, 151, 157, 163, 169, 181, 193, 211, 225, 229, 241, 277, 283, 289, 313, 331, 337, 361, 373, 397, 409, 421, 433, 457, 487, 523, 529, 541, 547, 577, 601, 613, 625, 631, 661, 673, 691, 709, 729, 733, 751, 757, 787
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OFFSET
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1,1
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LINKS
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EXAMPLE
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sigma(sigma(9)) = 14 but sigma(sigma(sigma(9))) = 24, so 9 is in the sequence.
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MATHEMATICA
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is3Q[n_]:=Boole[#>=2n&/@NestList[DivisorSigma[1, #]&, n, 3]]=={0, 0, 0, 1}; Select[Range[ 800], is3Q] (* Harvey P. Dale, Jul 09 2023 *)
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PROG
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(PARI) is(n) = {my(m = n, nn = 2*n, c = 0); while(m < nn, m = sigma(m); c++); c == 3; } \\ Amiram Eldar, Mar 18 2024
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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