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a(n) is the smallest integer that has exactly n divisors whose decimal digits are in strictly decreasing order.
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%I #14 Nov 03 2022 03:57:44

%S 1,2,4,6,12,20,30,40,80,60,252,120,240,540,360,630,420,960,1440,840,

%T 1260,2880,3360,4320,2520,6720,5040,8640,10080,15120,50400,20160,

%U 40320,30240,171360,90720,383040,60480,120960,181440,362880,544320,937440,786240,2056320

%N a(n) is the smallest integer that has exactly n divisors whose decimal digits are in strictly decreasing order.

%C This sequence is finite since A009995 is finite with 1022 nonzero terms, hence the last term is a(1022) = lcm of the 1022 positive terms of A009995.

%e For n=7, the divisors of 30 are {1, 2, 3, 5, 6, 10, 15, 30} of which 7 have their decimal digits in strictly decreasing order (all except 15). No integer < 30 has 7 such divisors, so a(7) = 30.

%t s[n_] := DivisorSum[n, 1 &, Greater @@ IntegerDigits[#] &]; seq[len_, nmax_] := Module[{v = Table[0, {len}], n = 1, c = 0, i}, While[c < len && n < nmax, i = s[n]; If[i <= len && v[[i]] == 0, v[[i]] = n; c++]; n++]; v]; seq[45, 3*10^6] (* _Amiram Eldar_, Nov 01 2022 *)

%o (PARI) f(n) = sumdiv(n, d, my(dd=digits(d)); vecsort(dd, , 12) == dd); \\ A358099

%o a(n) = my(k=1); while(f(k)!=n, k++); k; \\ _Michel Marcus_, Nov 01 2022

%Y Cf. A009995, A190219, A358099, A358101.

%Y Similar: A087997 (palindromic), A355303 (undulating), A357172 (increasing order).

%K nonn,base,fini

%O 1,2

%A _Bernard Schott_, Nov 01 2022

%E More terms from _Amiram Eldar_, Nov 01 2022