Palindromic number

A palindromic number is a symmetrical number written in some base a as a₁a₂a₃ ...|... a₃a₂a₁.

All numbers in base 10 with one digit {0, 1, 2, 3, 4, 5, 6, 7, 8, 9} are palindromic ones. The number of palindromic numbers with two digits is 9:

{11, 22, 33, 44, 55, 66, 77, 88, 99}.

There are 90 palindromic numbers with three digits:

{101, 111, 121, 131, 141, 151, 161, 171, 181, 191, ..., 909, 919, 929, 939, 949, 959, 969, 979, 989, 999}

and also 90 palindromic numbers with four digits:

{1001, 1111, 1221, 1331, 1441, 1551, 1661, 1771, 1881, 1991, ..., 9009, 9119, 9229, 9339, 9449, 9559, 9669, 9779, 9889, 9999},

so there are 199 palindromic numbers below 10⁴. Below 10⁵ there are 1099 palindromic numbers and for other exponents of 10ⁿ we have: 1999,10999,19999,109999,199999,1099999, ... (SIDN A070199). For some types of palindromic numbers these values are listed below in a table. Here 0 is included.

	101	102	103	104	105	106	107	108	109	1010
n natural	9	90		199	1099	1999	10999	19999	109999	199999
n even	5	9	49	89	489	+	+	+	+	+
n odd	5	10	60	110	610	+	+	+	+	+
n perfect square	3		6		13	14	19		+	+
n prime	4	5	20		113		781		5953
n square-free	6	12	67	120	675	+	+	+	+	+
n non-square-free (μ(n)=0)	3	6	41	78	423	+	+	+	+	+
n square with prime root	2		3		5
n with an even number of distinct prime factors (μ(n)=1)	2	6	35	56	324	+	+	+	+	+
n with an odd number of distinct prime factors (μ(n)=-1)	5	7	33	65	352	+	+	+	+	+
n even with an odd number of prime factors
n even with ann odd number of distinct prime factors	1	2	9	21	100	+	+	+	+	+
n odd with an odd number of prime factors	0	1	12	37	204	+	+	+	+	+
n odd with an odd number of distinct prime factors	0	0	4	24	139	+	+	+	+	+
n even squarefree with an even number of distinct prime factors	1	2	11	15	98	+	+	+	+	+
n odd squarefree with an even number of distinct prime factors	1	4	24	41	226	+	+	+	+	+
n odd with exactly 2 prime factors	1	4	25	39	205	+	+	+	+	+
n even with exactly 2 prime factors	2	3	11		64	+	+	+	+	+
n even with exactly 3 prime factors	1	3	14	24	122	+	+	+	+	+
n even with exactly 3 distinct prime factors
n odd with exactly 3 prime factors	0	1	12	34	173	+	+	+	+	+
n Carmichael number	0	0	0	0	0	1+	+	+	+	+
n for which σ(n) is palindromic	6	10	47	114	688	+	+	+	+	+
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