The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation. Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A061715 Numbers which are sandwiched between two numbers having the same ordered canonical form. 4
 4, 6, 12, 18, 30, 34, 42, 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, 248, 266, 270, 282, 300, 302, 304, 312, 320, 322, 328, 340, 348, 392, 394, 412, 414, 416, 420, 424, 432 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS The average of twin primes is a member. Is there ever a prime in the sequence? The sequence does not contain odd numbers since the odd number would be sandwiched between 2k and 2k+2 = 2(k+1) for some k and one of k, k+1 is odd and the other even so the highest power of two dividing them cannot be the same.  Since 2 is not in the sequence, there can be no primes. - Ray Chandler, Apr 13 2019 LINKS Amiram Eldar, Table of n, a(n) for n = 1..10000 EXAMPLE 34 is sandwiched between 33 and 35 which are of the form p*q where p and q are primes. MAPLE isA061715 := proc(n)     local nm1, np1 ;     nm1 := ifactors(n-1) ;     np1 := ifactors(n+1) ;     if nops(nm1) = nops(np1) then         for i from 1 to nops(nm1) do             if op(2, op(i, nm1)) <>  op(2, op(i, np1)) then                 return false;             end if;         end do:         true ;     else         false;     end if; end proc: for n from 1 to 300 do     if isA061715(n)  then         printf("%d, ", n);     end if; end do: # R. J. Mathar, Jan 18 2017 MATHEMATICA f[n_] := Flatten[Table[{ # []}] & /@ FactorInteger[n]]; Drop[ Select[ Range, Sort[f[ # - 1]] == Sort[f[ # + 1]] & ], 1] CROSSREFS Cf. A074497, A074498. Sequence in context: A130441 A068570 A074998 * A280469 A072570 A217259 Adjacent sequences:  A061712 A061713 A061714 * A061716 A061717 A061718 KEYWORD easy,nonn AUTHOR Amarnath Murthy, Aug 21 2002 EXTENSIONS Edited and extended by Robert G. Wilson v, Aug 22 2002 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified July 27 05:26 EDT 2021. Contains 346305 sequences. (Running on oeis4.)