The OEIS is supported by the many generous donors to the OEIS Foundation. Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 60th year, we have over 367,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”). Other ways to Give
 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A211223 Numbers k for which sigma(k) = sigma(x) + sigma(y), where k = x + y. 11
 3, 8, 9, 10, 15, 20, 21, 30, 32, 33, 39, 40, 49, 51, 55, 56, 57, 62, 63, 69, 70, 75, 85, 87, 88, 90, 92, 93, 94, 96, 99, 104, 105, 108, 110, 111, 114, 116, 117, 123, 125, 126, 128, 129, 130, 134, 135, 136, 140, 141, 145, 147, 150, 152, 153, 155, 158, 159, 160 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A211225(a(n)) > 0. - Reinhard Zumkeller, Jan 06 2013 LINKS Charles R Greathouse IV, Table of n, a(n) for n = 1..10000 EXAMPLE sigma(49) = sigma(8) + sigma(41) that is 57 = 15 + 42. sigma(93) = sigma(31) + sigma(62) that is 128 = 32 + 96. In more than one way: sigma(117) = sigma(41) + sigma(76) = sigma(52) + sigma(65) = sigma(56) + sigma(61) that is 182 = 42 + 140 = 98 + 84 = 120 + 62. MAPLE with(numtheory); A211223:=proc(q) local i, n; for n from 1 to q do for i from 1 to trunc(n/2) do if sigma(i)+sigma(n-i)=sigma(n) then print(n); break; fi; od; od; end: A211223(10000); MATHEMATICA sigmaPartitionQ[n_] := With[{s = DivisorSigma[1, n], ip = IntegerPartitions[ n, {2}]}, MemberQ[ip, {x_, y_} /; s == DivisorSigma[ 1, x] + DivisorSigma[ 1, y]]]; Select[Range, sigmaPartitionQ] (* Jean-François Alcover, Aug 19 2013 *) PROG (PARI) is(n)=my(t=sigma(n)); for(i=1, n\2, if(sigma(i)+sigma(n-i)==t, return(1))) \\ Charles R Greathouse IV, May 04 2012 (Haskell) a211223 n = a211223_list !! (n-1) a211223_list = map (+ 1) \$ findIndices (> 0) a211225_list -- Reinhard Zumkeller, Jan 06 2013 CROSSREFS Cf. A083207, A204830, A204831, A211224, A211225. Cf. A218852. Cf. A000203. Sequence in context: A091767 A024549 A051208 * A350776 A247544 A080524 Adjacent sequences: A211220 A211221 A211222 * A211224 A211225 A211226 KEYWORD nonn,nice AUTHOR Paolo P. Lava, Apr 27 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 | Recents
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified December 8 14:30 EST 2023. Contains 367679 sequences. (Running on oeis4.)