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A078492 No-prime decades. 3

%I

%S 200,320,510,530,620,840,890,1070,1130,1140,1260,1330,1340,1350,1410,

%T 1460,1500,1640,1670,1680,1710,1760,1790,1850,1890,1920,1960,2040,

%U 2070,2100,2180,2190,2320,2360,2400,2480,2490,2510,2560,2580,2820,2940,2980

%N No-prime decades.

%C Numbers n = 10*m such that none of n + 1, n + 3, n + 7, n + 9 are prime.

%C It is often possible to change the base 10 representation of a composite number into the representation of a prime by changing a single digit. That's not possible for the numbers in this sequence. - _Alonso del Arte_, Aug 25 2017

%H Daniel Starodubtsev, <a href="/A078492/b078492.txt">Table of n, a(n) for n = 0..10000</a>

%e Given that 201 = 3 * 67, 203 = 7 * 29, 207 = 3^2 * 23, 209 = 11 * 19, we conclude that 200 is in the sequence.

%e 210 is not in the sequence because 211 is prime.

%t Select[10Range[500], NextPrime[#] - # > 10 &] (* _Alonso del Arte_, Aug 25 2017 *)

%t Select[10 Range[300],NoneTrue[#+{1,3,7,9},PrimeQ]&] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jul 28 2020 *)

%o (PARI) decadenopr(n1,n2) = { forstep(x=n1,n2,10, if(!isprime(x+1) && !isprime(x+3) && !isprime(x+7) && !isprime(x+9), print1(x", "); ); ); }

%o (MAGMA) [10*n: n in [0..300] | forall{i: i in [1,3,7,9] | not IsPrime(10*n+i)}]; // _Bruno Berselli_, Jun 17 2016

%K nonn,base,easy

%O 0,1

%A _Cino Hilliard_, Jan 04 2003

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Last modified June 14 17:31 EDT 2021. Contains 345037 sequences. (Running on oeis4.)