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A074997 Numbers which are sandwiched between two numbers having the same unordered canonical form. 7
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

The average of twin primes is a member.

LINKS

Table of n, a(n) for n=1..56.

EXAMPLE

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

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);

MATHEMATICA

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

PROG

(PARI) isok(n) = vecsort(factor(n-1)[, 2]) == vecsort(factor(n+1)[, 2]); \\ Michel Marcus, Jul 28 2015

CROSSREFS

Cf. A074998, A061715, A074460.

Sequence in context: A022802 A301166 A310599 * A280383 A045959 A310600

Adjacent sequences:  A074994 A074995 A074996 * A074998 A074999 A075000

KEYWORD

nonn

AUTHOR

Amarnath Murthy, Aug 21 2002

EXTENSIONS

More terms from Sascha Kurz and Robert G. Wilson v, Aug 22 2002

Offset set to 1 by Michel Marcus, Jul 28 2015

STATUS

approved

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Last modified October 17 18:58 EDT 2019. Contains 328127 sequences. (Running on oeis4.)