Search: id:A020994
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%I A020994
%S A020994 2,3,5,7,23,37,53,73,313,317,373,797,3137,3797,739397
%N A020994 Primes that are both left-truncatable and right-truncatable.
%C A020994 Two-sided primes: deleting any number of digits at left or at right, 
               but not both, leaves a prime.
%C A020994 Primes in which every digit string containing the most significant digit 
               or the least significant digit is prime. - Amarnath Murthy (amarnath_murthy(AT)yahoo.com), 
               Sep 24 2003
%D A020994 Angell, I. O. and Godwin, H. J. "On Truncatable Primes." Math. Comput. 
               31, 265-267, 1977.
%D A020994 David Wells, The Penguin Dictionary of Curious and Interesting Numbers, 
               p. 178 (Rev. ed. 1997).
%H A020994 P. De Geest, <a href="http://www.worldofnumbers.com/truncat.htm">The 
               list of 4260 left-truncatable primes</a>
%H A020994 <a href="Sindx_Tri.html#tprime">Index entries for sequences related to 
               truncatable primes</a>
%t A020994 tspQ[n_] := Module[{idn=IntegerDigits[n], l}, l=Length[idn]; Union[PrimeQ/
               @(FromDigits/@ Join[Table[Take[idn, i], {i, l}], Table[Take[idn, 
               -i], {i, l}]])]=={True}] Select[Prime[Range[PrimePi[740000]]], tspQ]
%Y A020994 Cf. A033664, A024785, A032437, A024770, A052023, A052024, A052025, A050986, 
               A050987.
%Y A020994 Sequence in context: A104179 A096148 A124674 this_sequence A085823 A100552 
               A155873
%Y A020994 Adjacent sequences: A020991 A020992 A020993 this_sequence A020995 A020996 
               A020997
%K A020994 nonn,fini,full,base
%O A020994 1,1
%A A020994 Mario Velucchi (mathchess(AT)velucchi.it)
%E A020994 Corrected by David W. Wilson.
%E A020994 Additional comments from Harvey P. Dale (hpd1(AT)nyu.edu), Jul 10 2002

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