OFFSET
1,1
COMMENTS
There are 21 two-digit primes, hence n=1..21, and a(1)=21.
a(20)=0 because among 21 twenty-digit numbers no one is prime;
a(21)=0 because the only possible 42-digit number is composite:
978983797371676159534743413731292319171311=
2633*561934423224622169*661666288057389220943.
MATHEMATICA
pr2={97, 89, 83, 79, 73, 71, 67, 61, 59, 53, 47, 43, 41, 37, 31, 29, 23, 19, 17, 13, 11};
Table[ssk=Subsets[pr2, {k}]; Le=Binomial[21, k]; c=0;
Do[If[PrimeQ[fd=FromDigits[Flatten[IntegerDigits/@ssk[[i]]]]], c++ ], {i, Le}]; {k, c}, {k, 21}]
{1, 21}, {2, 36}, {3, 300}, {4, 722}, {5, 2493}, {6, 4766}, {7, 9459}, {8, 13091},
{9, 18484}, {10, 18779}, {11, 18048}, {12, 12995}, {13, 9122}, {14, 4424},
{15, 2160}, {16, 639}, {17, 234}, {18, 48}, {19, 8}, {20, 0}, {21, 0}.
CROSSREFS
KEYWORD
base,fini,full,nonn
AUTHOR
Zak Seidov, Nov 28 2009
STATUS
approved