A110778 Copies of 3 and 7 alternately such that every partial concatenation is a prime.

3, 7, 3, 777, 333, 777777777777777777777, 3333333, 7777777777777777777777777777777, 33333

Offset

1,1

Links

Table of n, a(n) for n=1..9.

Example

3, 37, 373, 373777, ... are all prime.

Mathematica

id[n_]:=IntegerDigits[n]; f[x_, y_]:=FromDigits[Flatten[Append[{x}, y]]]; a[x_, y_]:=NestWhile[f[id[#], y]&, f[id[x], y], !PrimeQ[#]&]; d[x_, y_]:=x-FromDigits[PadRight[id[y], Length[id[x]]]]; t={3}; x=3; Do[y=a[x, 7]; AppendTo[t, d[y, x]]; x=a[y, 3]; AppendTo[t, d[x, y]], {n, 4}]; t (* Jayanta Basu, May 20 2013 *)

Crossrefs

Cf. A110774, A110775, A110776, A110777.

Cf. A112534. - Sean A. Irvine, Mar 21 2010

Sequence in context: A125314 A213244 A050393 * A200129 A365729 A181912

Adjacent sequences: A110775 A110776 A110777 * A110779 A110780 A110781

Keyword

base,nonn

Author

Amarnath Murthy, Aug 12 2005

Extensions

More terms from Joshua Zucker, Jan 11 2006

The next term has 480 7's and is too large to include.

Status

approved