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A092619
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Numbers with property that number of prime digits is prime.
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1
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22, 23, 25, 27, 32, 33, 35, 37, 52, 53, 55, 57, 72, 73, 75, 77, 122, 123, 125, 127, 132, 133, 135, 137, 152, 153, 155, 157, 172, 173, 175, 177, 202, 203, 205, 207, 212, 213, 215, 217, 220, 221, 222, 223, 224, 225, 226, 227, 228, 229, 230, 231, 232, 233, 234
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OFFSET
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1,1
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COMMENTS
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A subset of A085557 from the 4th term.
Upper relative density in the primes is 1; lower relative density in the primes is 0. - Charles R Greathouse IV, Nov 14 2010
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LINKS
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EXAMPLE
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22 has two prime digits and their number 2 is prime, so 22 is a term.
222 has three prime digits and their number 3 is prime, so 222 is a term.
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MAPLE
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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:=proc(n) local i, stpf, ans, ans1; ans:=[ ]: stpf:=0: for i from 1 to n do if (isprime( ts_stpf(i) )='true') then ans:=[ op(ans), i ]: fi od; RETURN(ans) end: ts_pr(300);
# second Maple program:
q:= n-> isprime(nops(select(isprime, convert(n, base, 10)))):
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MATHEMATICA
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Select[Range[250], PrimeQ[Count[IntegerDigits[#], _?PrimeQ]]&] (* Harvey P. Dale, Nov 29 2011 *)
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PROG
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(Python)
from itertools import count, islice
from sympy import isprime
def A092619_gen(startvalue=1): # generator of terms >= startvalue
return filter(lambda n:isprime(sum(1 for d in str(n) if d in {'2', '3', '5', '7'})), count(max(startvalue, 1)))
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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