OFFSET
1,1
COMMENTS
The numbers such that sigma(k) = sigma(m) = m+k+1 and m+k is prime are in the sequence since sigma(k+m) = m+k+1 (see A005276). - Giovanni Resta, Mar 20 2014
LINKS
Giovanni Resta, Table of n, a(n) for n = 1..10000
EXAMPLE
The pair (1288, 1485) is in the sequence because sigma(1288) = sigma(1485) = 2880 and sigma(1288+1485) = sigma(2773) = 2880.
MATHEMATICA
a[n1_, n2_] := (t = Table[{DivisorSigma[1, n], n}, {n, n1, n2}] // Sort; s = Select[Split[t, #1[[1]] == #2[[1]] &], Length[#] >= 2 &]; f[lst_] := Select[Table[{lst[[i]], lst[[j]]}, {i, 1, Length[lst] - 1}, {j, i + 1, Length[lst]}] // Flatten[#, 1] &, #[[1, 1]] == DivisorSigma[1, #[[1, 2]] + #[[2, 2]]] &]; Select[f /@ s, # != {} &]); Flatten[a[1, 10^5], 2][[All, 2]] (* Jean-François Alcover, Mar 20 2014 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Michel Lagneau, Mar 18 2014
EXTENSIONS
More terms from Jean-François Alcover, Mar 20 2014
STATUS
approved