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%I #16 Apr 15 2024 03:28:32
%S 1,2,3,4,5,6,7,4,3,5,11,6,13,7,5,8,17,6,19,10,7,11,23,8,5,13,9,7,29,
%T 10,31,8,11,17,5,9,37,19,13,8,41,14,43,11,9,23,47,12,7,10,17,13,53,9,
%U 5,8,19,29,59,12,61,31,7,8,5,11,67,17,23,10,71,12,73,37,15,19,7,13,79,10,9
%N a(n) = the smallest divisor of n that is >= the number of positive divisors of n.
%H Amiram Eldar, <a href="/A138221/b138221.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = n for all primes plus the integers {1, 4, 6}. - _Robert G. Wilson v_
%e There are four positive divisors of 15: (1,3,5,15). The smallest of these divisors that is >=4 is 5; so a(15) = 5.
%p with(numtheory): a:=proc(n) local dn,i: dn:=divisors(n): for i while dn[i] < tau(n) do end do: dn[i] end proc: seq(a(n),n=1..60); # _Emeric Deutsch_, Mar 17 2008
%t f[n_] := First@Select[Divisors@n, # >= DivisorSigma[0, n] &]; Array[f, 81] (* _Robert G. Wilson v_ *)
%o (PARI) a(n) = {my(d = divisors(n), nd = #d); for(i = 1, nd, if(d[i] >= nd, return(d[i])));} \\ _Amiram Eldar_, Apr 15 2024
%Y Cf. A138222, A138223, A138224, A000005.
%K nonn,changed
%O 1,2
%A _Leroy Quet_, Mar 06 2008
%E More terms from _Emeric Deutsch_, _Robert G. Wilson v_ and _Erich Friedman_, Mar 17 2008
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