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A193881
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Numbers n such that 10^n-sigma(n^2) is prime.
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2
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OFFSET
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1,1
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COMMENTS
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sigma(x) is even unless x is a square or twice a square, therefore 10^n-sigma(n) can't be prime unless n is a square or twice a square, and {2, 49} are the only solutions < 10^4.
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LINKS
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MATHEMATICA
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Select[Range[1000], PrimeQ[10^# - DivisorSigma[1, #^2]] &] (* Robert Price, Mar 24 2015 *)
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PROG
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(PARI) for(n=1, 9999, ispseudoprime(t=10^n-sigma(n^2)) && print1(n", "))
(Magma) [n: n in [1..450] | IsPrime(10^n-DivisorSigma(1, n^2))]; // Vincenzo Librandi, Mar 26 2015
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CROSSREFS
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KEYWORD
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nonn,hard
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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