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 A063806 Numbers with a prime number of proper divisors. 4
 4, 6, 8, 9, 10, 12, 14, 15, 18, 20, 21, 22, 24, 25, 26, 27, 28, 30, 32, 33, 34, 35, 38, 39, 40, 42, 44, 45, 46, 49, 50, 51, 52, 54, 55, 56, 57, 58, 60, 62, 63, 65, 66, 68, 69, 70, 72, 74, 75, 76, 77, 78, 82, 84, 85, 86, 87, 88, 90, 91, 92, 93, 94, 95, 96, 98, 99, 102, 104 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harry J. Smith, Table of n, a(n) for n = 1..1000 EXAMPLE 15 has three proper divisors {1,3,5} and so is on the list; 16 has four {1,2,4,8} and so is not; 17 has one {1} and so is not; 18 has five {1,2,3,6,9} and so is. PROG (PARI) n=0; for (m=1, 10^9, if(isprime(numdiv(m) - 1), write("b063806.txt", n++, " ", m); if (n==1000, break))) \\ Harry J. Smith, Aug 31 2009 (PARI) is(n)=isprime(numdiv(n)-1) \\ Charles R Greathouse IV, Sep 18 2015 CROSSREFS A subset of A002808. Cf. A009087, A032741. Sequence in context: A117098 A077136 A098216 * A323521 A063989 A168645 Adjacent sequences:  A063803 A063804 A063805 * A063807 A063808 A063809 KEYWORD nonn AUTHOR Henry Bottomley, Aug 20 2001 STATUS approved

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Last modified April 19 08:44 EDT 2019. Contains 322241 sequences. (Running on oeis4.)