A111707 a(n) = Sum_{k = 1..ceiling(w/2)} d(k) * d(w+1-k), where (d(1), ..., d(w)) is the decimal expansion of n.

0, 1, 4, 9, 16, 25, 36, 49, 64, 81, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 0, 4, 8, 12, 16, 20, 24, 28, 32, 36, 0, 5, 10, 15, 20, 25, 30, 35, 40, 45, 0, 6, 12, 18, 24, 30, 36, 42, 48, 54, 0, 7, 14, 21, 28, 35, 42, 49, 56, 63, 0, 8

Offset

0,3

Links

Rémy Sigrist, Table of n, a(n) for n = 0..10000

Example

a(12) = 1*2 = 2, a(12345) = 1*5 + 2*4 + 3*3 = 22.

Mathematica

Array[Sum[#1[[k]]*#1[[#2 + 1 - k]], {k, Ceiling[#2/2]}] & @@ {IntegerDigits[#], IntegerLength[#]} &, 82, 0] (* Michael De Vlieger, May 21 2021 *)

Prog

(PARI) a(n) = { my (d=digits(n)); sum (k=1, ceil(#d/2), d[k]*d[#d+1-k]) } \\ Rémy Sigrist, May 21 2021

Crossrefs

Cf. A085942.

Sequence in context: A072595 A334832 A169669 * A115300 A085942 A066308

Adjacent sequences: A111704 A111705 A111706 * A111708 A111709 A111710

Keyword

base,easy,nonn

Author

Amarnath Murthy, Aug 22 2005

Extensions

More terms from Joshua Zucker, May 08 2006

a(0) = 0 prepended and name clarified by Rémy Sigrist, May 21 2021

Status

approved