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A124665 Numbers that cannot be either prefixed or followed by one digit to form a prime. 4
20, 32, 62, 84, 114, 126, 134, 135, 146, 150, 164, 168, 176, 185, 192, 196, 204, 210, 218, 232, 236, 240, 248, 256, 258, 282, 294, 298, 305, 314, 315, 324, 326, 328, 342, 348, 350, 356, 366, 368, 374, 375, 378, 395, 406, 408, 410, 414, 416, 418 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Prefixing by 0 gives the number itself, implying that a(n) is not prime.

Intersection of A065502 and A032352. - Reinhard Zumkeller, Oct 22 2011

LINKS

Reinhard Zumkeller, Table of n, a(n) for n = 1..10000

EXAMPLE

If you prefix 20 with any digit you will get an even number. Also 201, 203, 207 and 209 are all composite.

MATHEMATICA

okQ[n_]:=If[EvenQ[n]||Divisible[n, 5], Union[PrimeQ[10 n+{1, 3, 7, 9}]] == {False}, !PrimeQ[n]&&Union[PrimeQ[10 n+{1, 3, 7, 9}]]=={False} && Union[ PrimeQ[Table[FromDigits[Join[{i}, IntegerDigits[n]]], {i, 9}]]] == {False}]; Select[Range[500], okQ] (* Harvey P. Dale, Jul 15 2011 *)

PROG

(PARI) a(n)=my(N=10*n, D=10^#Str(n)); forstep(k=n, n+9*D, D, if(isprime(k), return(0))); !(isprime(N+1)||isprime(N+3)||isprime(N+7)||isprime(N+9)) \\ Charles R Greathouse IV, Jul 15 2011

(Haskell)

a124665 n = a124665_list !! (n-1)

a124665_list = filter

   (\x -> all (== 0) $ map (a010051 . (10*x +)) [1..9]) a065502_list

-- Reinhard Zumkeller, Oct 22 2011

CROSSREFS

Cf. A010051, A032737, A125268.

Sequence in context: A075035 A032352 A183048 * A134989 A119873 A075230

Adjacent sequences:  A124662 A124663 A124664 * A124666 A124667 A124668

KEYWORD

base,nonn

AUTHOR

Tanya Khovanova, Dec 23 2006

STATUS

approved

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Last modified February 26 16:30 EST 2017. Contains 282689 sequences.