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 A335709 a(n) is the smallest Niven number that has exactly n divisors or -1 if no such number exists. 1
 1, 2, 4, 6, 81, 12, -1, 24, 36, 48, 59049, 60, -1, 192, 144, 120, 43046721, 180, 43472473122830653562489222659449707872441, 240, 576, 3072, 191540580003116921429323712183642218614831262597249, 360, 1296, 94208, 900, 960, -1, 720, -1, 840, 9216, 720896, 5184, 1260 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS If n is a prime number, then a(n) has the form p^(n-1), where p is a prime number such that p <= 9 * ((n-1) * log_10(p) +1). For p <= 9 * ((n-1) * log_10 (p) +1), if there is no s >= 1 such that digsum(p^(n-1)) = p^s, then a(n) = -1. For example, for n = 7, the largest prime number p verifying p <= 9 * (6 * log_10 (p) +1) is 113, but no prime number q <= 113 has the property digsum(q^6) = q^s, for 1 <= s <= 6. Thus, a(7) = -1. LINKS EXAMPLE The number 81 = 3^4 is the smallest with 5 divisors and is a Niven number, so a(5) = 81. PROG (MAGMA) niven:=func; a:=[]; for n in [1..36] do if not IsPrime(n) then k:=1; while not niven(k) or #Divisors(k) ne n do k:=k+1; end while; Append(~a, k); else q:=2; while not niven(q^(n-1)) and q le (9*(n-1)*Log(10, q)+9) do q:=NextPrime(q); end while; if niven(q^(n-1)) then Append(~a, q^(n-1)); else Append(~a, -1); end if; end if; end for; a; CROSSREFS Cf. A000005, A005349, A007953. Sequence in context: A066220 A009257 A098757 * A056012 A259050 A066719 Adjacent sequences:  A335706 A335707 A335708 * A335710 A335711 A335712 KEYWORD sign,base AUTHOR Marius A. Burtea, Aug 04 2020 STATUS approved

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Last modified September 28 19:12 EDT 2021. Contains 347717 sequences. (Running on oeis4.)