A074997 Numbers which are sandwiched between two numbers having the same unordered canonical form.
4, 6, 12, 18, 19, 30, 34, 42, 51, 55, 56, 60, 72, 86, 92, 94, 102, 108, 138, 142, 144, 150, 160, 180, 184, 186, 192, 198, 202, 204, 214, 216, 218, 220, 228, 236, 240, 243, 248, 249, 266, 270, 282, 300, 302, 304, 307, 312, 320, 322, 328, 340, 341, 348, 349, 392
Offset: 1
Keywords
Examples
19 is sandwiched between 18 and 20 which are of the form p^2*q where p and q are primes.
Links
- Amiram Eldar, Table of n, a(n) for n = 1..10000
Programs
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Maple
k := 0: for j from 2 to 5000 do a := ifactors(j-1): b := ifactors(j+1): if sort([seq(a[2][i][2],i= 1..nops(a[2]))])= sort([seq(b[2][i][2],i= 1..nops(b[2]))]) then k := k+1: c[k] := j: fi: od: seq(c[i],i= 1..k);
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Mathematica
f[n_] := Flatten[Table[{ # [[2]]}] & /@ FactorInteger[n]]; Drop[ Select[ Range[415], Sort[f[ # - 1]] == Sort[f[ # + 1]] & ], 1] -
PARI
isok(n) = vecsort(factor(n-1)[,2]) == vecsort(factor(n+1)[,2]); \\ Michel Marcus, Jul 28 2015
Extensions
More terms from Sascha Kurz and Robert G. Wilson v, Aug 22 2002
Offset set to 1 by Michel Marcus, Jul 28 2015
Comments