A077377 Squarefree numbers whose external digits form a squarefree number.

1, 2, 3, 5, 6, 7, 10, 11, 13, 14, 15, 17, 19, 21, 22, 23, 26, 29, 30, 31, 33, 34, 35, 37, 38, 39, 41, 42, 43, 46, 47, 51, 53, 55, 57, 58, 59, 61, 62, 65, 66, 67, 69, 70, 71, 73, 74, 77, 78, 79, 82, 83, 85, 86, 87, 89, 91, 93, 94, 95, 97, 101, 103, 105, 107, 109, 110, 111, 113

Offset: 1

Author

Amarnath Murthy, Nov 06 2002

Keywords

• base
• easy
• nonn

Links

• Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A005117, A077376, A077378, A077379, A077380.

Programs

Mathematica

A077377Q[k_] := SquareFreeQ[k] && (k < 10 || SquareFreeQ[FromDigits[IntegerDigits[k][[{1, -1}]]]]);
Select[Range[200], A077377Q] (* Paolo Xausa, Feb 19 2025 *)

PARI

list(lim)=my(v=List(),n,t); for(d=0,logint(lim\=1,10), t=10^d; forsquarefree(k=t,min(10*t-1,lim), if(issquarefree(k[1]\t*10 + k[1]%10), listput(v,k[1])))); Vec(v) \\ Charles R Greathouse IV, Feb 14 2018

Extensions

a(1) inserted by Charles R Greathouse IV, Feb 14 2018