a(n) = Sum_{k=1..n} k^(2^(n+1-k)).A140423a(n) = Sum_{k=1..n} k^(2^(n+1-k)).Terms a(0) =1a(1) =5a(2) =26a(3) =354a(4) =72379a(5) =4338080215External referencesoeis: A140423