A219998 Prime-free decades such that the previous decade is also prime-free.

114, 134, 135, 168, 219, 249, 315, 324, 348, 375, 419, 431, 477, 485, 513, 525, 537, 546, 561, 576, 597, 641, 651, 675, 693, 709, 714, 727, 738, 777, 798, 834, 840, 848, 849, 891, 908, 936, 945, 957, 999, 1002, 1005, 1038, 1041, 1081, 1082, 1092, 1134, 1176

Offset: 1

Author

V. Raman, Dec 07 2012

Keywords

• nonn
• base

Comments

Numbers n such that 10n-9, 10n-7, 10n-3, 10n-1, 10n+1, 10n+3, 10n+7, and 10n+9 are composite.

Links

• V. Raman, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A190639, A032352.

Cf. A216288 (lower decade).

Programs

PARI

for(i=2, 1200, if(isprime(10*i-9)==0&&isprime(10*i-7)==0&&isprime(10*i-3)==0&&isprime(10*i-1)==0&&isprime(10*i+1)==0&&isprime(10*i+3)==0&&isprime(10*i+7)==0&&isprime(10*i+9)==0, print1(i", ")))

Formula

a(n) ~ n.

a(n) = A216288(n) + 1.