A250071 - OEIS

A250071

Smallest number k such that the symmetric representation of sigma(k) has maximum width n for those k whose representation has nondecreasing width up to the diagonal.

1, 6, 72, 120, 5184, 1440, 373248, 6720, 28800, 103680, 1934917632, 80640, 278628139008, 7464960, 2073600, 483840, 1444408272617472, 1612800, 103997395628457984, 5806080, 298598400, 77396705280, 539122498937926189056, 7096320, 1658880000, 5572562780160, 90316800, 418037760, 402452788967166148425547776, 116121600

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OFFSET

1,2

COMMENTS

The symmetric representation of sigma(k) has nondecreasing width to the diagonal precisely when all odd divisors counted in the k-th row of A237048 occur at odd indices. If we write k = 2^m * q with m >= 0 and q odd, this property is equivalent to q < 2^(m+1).

The values for a(11), a(13), a(17) and a(19) were computed directly using the formula k = 2^m * 3^(p-1) where p is one of the four primes and m the smallest exponent so that 3^(p-1) < 2^(m+1). Each of these numbers has a symmetric representation of nondecreasing width ending in a prime number width, and they are the first such numbers since the number of divisors of an odd number is a prime precisely when the number is a power of a prime.

The other numbers listed whose symmetric representations of sigma(k) have nondecreasing width are smaller than 7500000. The only additional numbers k <= 100000000 are a(24) = 7096320, a(27) = 90316800 and a(32) = 85155840.

See A340506 for another way to look at this data. - N. J. A. Sloane, Jan 23 2021

LINKS

Table of n, a(n) for n=1..30.

FORMULA

a(n) = min(2^m * q, m >= 0 & q odd & sigma_0(q) = n & q < 2^(m+1)) where sigma_0 is the number of divisors.

a(p) = 2^ceiling((p-1)*(log_2(3)) - 1) * 3^(p-1) for primes p.

EXAMPLE

a(6) = 1440 = 2^5 * 3^2 * 5 has 6 odd divisors. It is the smallest number of the form 2^m * q with m > 0, q odd and such that q < 2^(m+1).

MATHEMATICA

(* function a2[ ] is defined in A249223 *)

smallQ[n_] := Module[{x=2^IntegerExponent[n, 2]}, n/x<2x]

ndWidth[{m_, n_}] := Select[Range[m, n], smallQ]

a250071[x_List] := Module[{i, max, acc={{1, 1}}}, For[i=1, i<=Length[x], i++, max={Max[a2[x[[i]]]], x[[i]]}; If[!MemberQ[Transpose[acc][[1]], max[[1]]], AppendTo[acc, max]]]; acc]

(* returns (argument, result) data pairs since sequence is non-monotonic *)

Sort[a250071[ndWidth[{1, 100000000}]]] (* computed in steps *)

(* alternate implementation using function f[ ] by T. D. Noe in A162247 *)

sF[n_] := Min[Map[Apply[Times, Prime[Range[2, Length[#]+1]]^#]&, Map[Reverse[#-1]&, f[n]]]]

f1U[n_] := Module[{s=sF[n], k}, k=Floor[Log[2, s]]; 2^k s]

a250071[n_] := Map[f1U, Range[n]]

a250071[30] (* Hartmut F. W. Hoft, Nov 27 2024 *)

CROSSREFS