A053332 a(n) contains n digits (either '4' or '7') and is divisible by 2^n.

4, 44, 744, 7744, 47744, 447744, 4447744, 44447744, 444447744, 4444447744, 74444447744, 474444447744, 4474444447744, 44474444447744, 444474444447744, 7444474444447744, 77444474444447744, 477444474444447744

Offset

1,1

Links

Ray Chandler, Table of n, a(n) for n = 1..1000

Formula

a(n) = a(n-1)+10^(n-1)*(4+3*[a(n-1)/2^(n-1) mod 2]) i.e. a(n) ends with a(n-1); if (n-1)-th term is divisible by 2^n then n-th term begins with a 4, if not then n-th term begins with a 7.

Mathematica

Flatten[Table[Select[FromDigits/@Tuples[{4, 7}, n], Divisible[#, 2^n]&], {n, 20}]] (* Harvey P. Dale, Jul 25 2011 *)

Crossrefs

Cf. A023404, A050621, A050622, A035014.

Sequence in context: A218224 A177749 A291198 * A276369 A157093 A365556

Adjacent sequences: A053329 A053330 A053331 * A053333 A053334 A053335

Keyword

base,nonn

Author

Henry Bottomley, Mar 06 2000

Status

approved