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A074997 Numbers which are sandwiched between two numbers having the same unordered canonical form. 8

%I #13 Jan 02 2020 04:15:13

%S 4,6,12,18,19,30,34,42,51,55,56,60,72,86,92,94,102,108,138,142,144,

%T 150,160,180,184,186,192,198,202,204,214,216,218,220,228,236,240,243,

%U 248,249,266,270,282,300,302,304,307,312,320,322,328,340,341,348,349,392

%N Numbers which are sandwiched between two numbers having the same unordered canonical form.

%C The average of twin primes is a member.

%H Amiram Eldar, <a href="/A074997/b074997.txt">Table of n, a(n) for n = 1..10000</a>

%e 19 is sandwiched between 18 and 20 which are of the form p^2*q where p and q are primes.

%p 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);

%t f[n_] := Flatten[Table[{ # [[2]]}] & /@ FactorInteger[n]]; Drop[ Select[ Range[415], Sort[f[ # - 1]] == Sort[f[ # + 1]] & ], 1]

%o (PARI) isok(n) = vecsort(factor(n-1)[,2]) == vecsort(factor(n+1)[,2]); \\ _Michel Marcus_, Jul 28 2015

%Y Cf. A074998, A061715, A074460.

%K nonn

%O 1,1

%A _Amarnath Murthy_, Aug 21 2002

%E More terms from _Sascha Kurz_ and _Robert G. Wilson v_, Aug 22 2002

%E Offset set to 1 by _Michel Marcus_, Jul 28 2015

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Last modified March 29 05:48 EDT 2024. Contains 371265 sequences. (Running on oeis4.)