This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A213025 Balanced semiprimes (of order one): semiprimes which are the average of the previous semiprime and the following semiprime. 4
 34, 86, 94, 122, 142, 185, 194, 202, 214, 218, 262, 289, 302, 314, 321, 358, 371, 394, 407, 413, 415, 422, 446, 471, 489, 493, 497, 517, 535, 562, 581, 586, 626, 634, 669, 687, 698, 734, 785, 791, 815, 838, 842, 922, 982, 989, 1042, 1057, 1079, 1135, 1138 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Semiprimes that are the average of three successive semiprimes. First term not also in A086005 is 185. - Alonso del Arte, Jun 04 2012 LINKS Alois P. Heinz, Table of n, a(n) for n = 1..10000 Eric Weisstein's World of Mathematics, Semiprime FORMULA 2*sp_(n) = sp_(n - 1) + sp_(n + 1). a(n) = 1/3 * (sp(i) + sp(i + 1) + sp(i + 2), for some i(n). EXAMPLE 194 is in the sequence because 194 = (187 + 194 + 201)/3 = (A001358(61) + A001358(62) + A001358(63))/3. MAPLE with(numtheory): prevsp:= proc(n) local k; for k from n-1 by -1            while isprime(k) or bigomega(k)<>2 do od; k end: nextsp:= proc(n) local k; for k from n+1            while isprime(k) or bigomega(k)<>2 do od; k end: a:= proc(n) option remember; local s;       s:= `if`(n=1, 4, a(n-1));       do s:= nextsp(s);          if s=(prevsp(s)+nextsp(s))/2 then break fi       od; s     end: seq (a(n), n=1..100);  # Alois P. Heinz, Jun 03 2012 MATHEMATICA bspQ[{a_, b_, c_}]:=b==(a+c)/2; With[{sp=Select[Range[1200], PrimeOmega[#] == 2&]}, Transpose[Select[Partition[sp, 3, 1], bspQ]][[2]]] (* Harvey P. Dale, Nov 18 2012 *) PROG (Haskell) a213025 n = a213025_list !! (n-1) a213025_list = f a001358_list where    f (x:sps'@(y:z:sps)) | 2 * y == (x + z) = y : f sps'                         | otherwise        = f sps' -- Reinhard Zumkeller, Jun 10 2012 CROSSREFS Cf. A086005 (subsequence), A001358, A006562, A065516, A212820. Sequence in context: A046764 A260276 A278311 * A086005 A169834 A248201 Adjacent sequences:  A213022 A213023 A213024 * A213026 A213027 A213028 KEYWORD nonn AUTHOR Gerasimov Sergey, Jun 03 2012 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 August 22 20:43 EDT 2019. Contains 326198 sequences. (Running on oeis4.)