A069845 - OEIS

%I #8 Aug 24 2019 11:52:55

%S 1,0,1,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,2,0,2,0,0,0,0,0,0,0,0,0,1,0,1,0,

%T 0,0,1,0,2,0,0,0,1,0,0,0,0,0,1,0,1,0,0,0,0,0,2,0,0,0,2,0,2,0,0,0,2,0,

%U 0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0,2,0,0,0,0,0,1,0,1,0,0,0,0,0,0

%N Number of distinct primes obtained by inserting a 4 at all possible places in n. This includes prefixing.

%H Antti Karttunen, <a href="/A069845/b069845.txt">Table of n, a(n) for n = 1..20000</a>

%e See the example in A069842.

%t Table[ Count[ PrimeQ[ Union[ FromDigits /@ Table[ Insert[ IntegerDigits[n], 4, j], {j, 1, Floor[ Log[10, n] + 2]}]]], True], {n, 1, 105}]

%o (PARI) A069845(n) = { my(digs=digits(n), u=#digs, nums=Set([])); for(n=1,1+u, nums = setunion(nums,[fromdigits(vector(1+u,i,if(i<n,digs[i],if(i>n,digs[i-1],4))))])); #select(isprime,nums); }; \\ _Antti Karttunen_, Aug 23 2019

%Y Cf. A069842, A069843, A069844, A069846, A069847, A069848, A069849, A069850 and A069851.

%K base,nonn

%O 1,19

%A _Amarnath Murthy_, Apr 16 2002

%E Edited and extended by _Robert G. Wilson v_, Apr 18 2002