A112534 Number of digits in A110778(n).

1, 1, 1, 3, 3, 21, 7, 31, 5, 480, 1526, 294, 1615

Offset: 1

Author

Joshua Zucker, Jan 12 2006

Keywords

• base
• nonn

Examples

			A110778(4) = 777 hence a(4) = 3.
		

Crossrefs

Cf. A110774, A110775, A110776, A110777, A110778, A110780, A110781, A110782, A110783, A110788, A110789.

Programs

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[#]&]; le[n_]:=Length[id[n]]; d[x_,y_]:=x-FromDigits[PadRight[id[y],le[x]]]; x=3; t={1}; Do[y=a[x,7]; AppendTo[t,le[d[y,x]]]; x=a[y,3]; AppendTo[t,le[d[x,y]]],{n,5}]; t (* Jayanta Basu, May 20 2013 *)

Python

from sympy import isprime
from itertools import count, islice
def agen(): # generator of terms
    s = ""
    while True:
        for d in "37":
            for k in count(1):
                if isprime(int(s+d*k)): break
            yield k
            s += d*k
print(list(islice(agen(), 10))) # Michael S. Branicky, Aug 23 2022

Extensions

Typo in example fixed by Sean A. Irvine, Mar 21 2010

a(11)-a(12) from Sean A. Irvine, Mar 22 2010

a(13) from Michael S. Branicky, Aug 23 2022