A092622 Primes with exactly two prime digits.

23, 37, 53, 73, 127, 137, 157, 173, 229, 239, 251, 263, 271, 283, 293, 307, 313, 317, 331, 347, 359, 367, 379, 383, 397, 433, 457, 503, 521, 547, 563, 571, 587, 593, 653, 673, 677, 739, 743, 751, 787, 797, 823, 827, 853, 857, 877, 937, 953, 977, 1033, 1123

Offset

1,1

Links

Table of n, a(n) for n=1..52.

Formula

a(n) >> n^1.285

Example

23 is prime and it has two prime digits, 2 and 3;
127 is prime and it has two prime digits 2 and 7.

Maple

stev_sez:=proc(n) local i, tren, st, ans, anstren; ans:=[ ]: anstren:=[ ]: tren:=n: for i while (tren>0) do st:=round( 10*frac(tren/10) ): ans:=[ op(ans), st ]: tren:=trunc(tren/10): end do; for i from nops(ans) to 1 by -1 do anstren:=[ op(anstren), op(i, ans) ]; od; RETURN(anstren); end: ts_stpf:=proc(n) local i, stpf, ans; ans:=stev_sez(n): stpf:=0: for i from 1 to nops(ans) do if (isprime(op(i, ans))='true') then stpf:=stpf+1; # number of prime digits fi od; RETURN(stpf) end: ts_pr_prnd:=proc(n) local i, stpf, ans, ans1, tren; ans:=[ ]: stpf:=0: tren:=1: for i from 1 to n do if ( isprime(i)='true' and ts_stpf(i) = 2) then ans:=[ op(ans), i ]: tren:=tren+1; fi od; RETURN(ans) end: ts_pr_prnd(2500);

Crossrefs

Cf. A034844.

Sequence in context: A163759 A190731 A348699 * A129351 A179910 A124888

Adjacent sequences: A092619 A092620 A092621 * A092623 A092624 A092625

Keyword

nonn,base

Author

Jani Melik, Apr 11 2004

Status

approved