A098731 - OEIS

%I #15 Aug 02 2023 20:53:33

%S 7,8,11,17,25,44,47,48,52,59,70,80,89,92,94,98,101,110,116,124,134,

%T 139,152,158,161,164,170,175,179,184,187,196,205,209,215,222,226,228,

%U 233,235,241,250,269,272,273,278,282,287,288,296,297,303,322,325,333,361

%N Numbers k such that 2*R_k is a happy number (A007770), where R_k = 11...1 is the repunit (A002275) of length k.

%H Chai Wah Wu, <a href="/A098731/b098731.txt">Table of n, a(n) for n = 1..10000</a>

%e 2*R_11 = 2*11111111111 = 22222222222 is a happy number, so 11 is in the list.

%o (Python)

%o from itertools import count, islice

%o def A098731_gen(startvalue=1): # generator of terms

%o     for n in count(max(startvalue,1)):

%o         m = n<<2

%o         while m not in {1,37,58,89,145,42,20,4,16}:

%o             m = sum((0, 1, 4, 9, 16, 25, 36, 49, 64, 81)[ord(d)-48] for d in str(m))

%o         if m == 1:

%o             yield n

%o A098731_list = list(islice(A098731_gen(),20)) # _Chai Wah Wu_, Aug 02 2023

%Y Cf. A007770, A002275.

%K easy,nonn,base

%O 1,1

%A Lambert Klasen (lambert.klasen(AT)gmx.de), Oct 28 2004