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A086557 Smallest number not included earlier, beginning with 2 and having prime signature of n. 0
2, 23, 25, 29, 21, 211, 27, 289, 22, 223, 20, 227, 26, 201, 2401, 229, 28, 233, 207, 202, 203, 239, 24, 2209, 205, 2197, 212, 241, 222, 251, 243, 206, 209, 213, 225, 257, 214, 215, 232, 263, 230, 269, 236, 242, 217, 271, 208, 2809, 244, 218, 245, 277, 248, 219 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,1

COMMENTS

A rearrangement of numbers beginning with 2.

LINKS

Table of n, a(n) for n=2..55.

EXAMPLE

a(40) = 232 = 2^3 * 29 as 40 = 2^3 * 5.

MAPLE

se := {seq(20+k, k=0..9), seq(200+k, k=0..99), seq(2000+k, k=0..999), seq(20000+k, k=0..9999), seq(200000+k, k=0..99999)}: a[2] := 2:for n from 3 to 60 do l := sort(convert(se, list)): ifa := ifactors(n):no := nops(ifa[2]): prisig := sort([seq(ifa[2][j][2], j=1..no)]):h := 0:while(true) do h := h+1: ifa := ifactors(l[h]): no2 := nops(ifa[2]):prisig2 := sort([seq(ifa[2][j][2], j=1..no2)]):ok := true: if(no<>no2) then ok := false: else for w from 1 to no do if(prisig[w]<>prisig2[w]) then ok := false:break:fi:od:fi: if(ok=true) then a[n] := l[h]:se := se minus {l[h]}:break:fi:od:od:seq(a[q], q=2..60); # Sascha Kurz

MATHEMATICA

Block[{a = {}, s = PositionIndex@ Table[ToString@ Sort[FactorInteger[n][[All, -1]]], {n, Product[Prime@ i, {i, 6}]}]}, Do[AppendTo[a, #] &@ SelectFirst[Lookup[s, ToString@ Sort[FactorInteger[n][[All, -1]]] ], And[First@ IntegerDigits@ # == 2, FreeQ[a, #]] &], {n, 2, 55}]; a] (* Michael De Vlieger, Aug 15 2017 *)

CROSSREFS

Cf. A086549.

Sequence in context: A131983 A153718 A105813 * A042797 A041649 A141533

Adjacent sequences:  A086554 A086555 A086556 * A086558 A086559 A086560

KEYWORD

base,nonn

AUTHOR

Amarnath Murthy, Aug 30 2003

EXTENSIONS

Corrected and extended by Sascha Kurz, Sep 22 2003

STATUS

approved

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Last modified August 26 03:38 EDT 2019. Contains 326324 sequences. (Running on oeis4.)