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 A221647 Smallest number k such that prime(n) is the n-th divisor of k. 3
 3, 10, 28, 66, 234, 204, 456, 828, 1392, 2232, 2220, 5904, 7224, 5640, 9540, 14160, 14640, 28140, 25560, 26280, 79632, 89640, 64080, 69840, 181800, 129780, 134820, 183120, 189840, 213360, 495180, 460320, 934080, 1001280, 380520, 1243440, 1779960 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS The similar problem "smallest number k such that prime(n) is the n-th prime divisor of k" is given by the sequence A002110: primorial numbers product of first n primes. LINKS Donovan Johnson, Table of n, a(n) for n = 2..300 EXAMPLE a(6) = 234 because the divisors of 234 are {1, 2, 3, 6, 9, 13, 18, 26, 39, 78, 117, 234}, and prime(6) = 13 is the 6th divisor of 234. MAPLE A221647 := proc(n)         local p, k, j ;         p := ithprime(n) ;         for j from 1 do                 k := j*p ;                 dvs := sort(convert(numtheory[divisors](k), list)) ;                 if nops(dvs) >= n then                 if op(n, dvs) = p then                         return k ;                 end if;                 end if;         end do: end proc: seq(A221647(n), n=2..30) ; # R. J. Mathar, May 05 2013 MATHEMATICA nn = 20; t = Table[0, {nn}]; found = 1; n = 2; While[found < nn, n++; d = Divisors[n]; Do[If[i <= nn && d[[i]] == Prime[i] && t[[i]] == 0, t[[i]] = n; found++], {i, Length[d]}]]; Rest[t] (* T. D. Noe, May 07 2013 *) CROSSREFS Cf. A000040, A002110, A027750. Sequence in context: A325800 A037167 A267707 * A062431 A034351 A182737 Adjacent sequences:  A221644 A221645 A221646 * A221648 A221649 A221650 KEYWORD nonn AUTHOR Michel Lagneau, May 04 2013 STATUS approved

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Last modified September 20 05:51 EDT 2019. Contains 327212 sequences. (Running on oeis4.)