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 A048703 Numbers which in base 4 are palindromes and have an even number of digits. 4
 0, 5, 10, 15, 65, 85, 105, 125, 130, 150, 170, 190, 195, 215, 235, 255, 1025, 1105, 1185, 1265, 1285, 1365, 1445, 1525, 1545, 1625, 1705, 1785, 1805, 1885, 1965, 2045, 2050, 2130, 2210, 2290, 2310, 2390 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS In quaternary base (base 4) the terms look like 0, 11, 22, 33, 1001, 1111, 1221, 1331, 2002, 2112, 2222, 2332, 3003, 3113, 3223, 3333, 100001, 101101, 102201, ..., which is a subsequence of A118595. Zero is included as a(0) because we can consider it as having zero digits after leading zeros have been excluded. LINKS Antti Karttunen, Table of n, a(n) for n = 0..4095 FORMULA a(0) = 0, and for n >= 1, a(n) = A030103(n) + (n * A000302(A110591(n))). - Antti Karttunen, Oct 30 2013 a(n) = 5*A048704(n). [This is just a consequence of the definition of A048704.] - Antti Karttunen, Jul 25 2013 EXAMPLE Each a(n) is obtained by concatenating the original base-4 expansion of n (which comes to the left hand, i.e., the most significant side) with its mirror-image (which comes to the right hand, i.e., the least significant side). For example, for a(4) we have 4 = '10' in base 4, which concatenated with its reversal '01' yields '1001', which when converted back to decimal yields 1*64 + 0*16 + 0*4 + 1*1 = 65, thus a(4)=65. MAPLE A048703(n) := (n) -> (2^(floor_log_2_coarse(n)+1))*n + sum('(bit_i(n, i+((-1)^i))*(2^(floor_log_2_coarse(n)-i)))', 'i'=0..floor_log_2_coarse(n)); bit_i := (x, i) -> `mod`(floor(x/(2^i)), 2); # Following is like floor_log_2 but even results are incremented by one: floor_log_2_coarse := proc(n) local nn, i: nn := n; for i from -1 to n do if(0 = nn) then RETURN(i+(1-(i mod 2))); fi: nn := floor(nn/2); od: end: PROG (MIT/GNU Scheme) (define (A048703 n) (if (zero? n) n (let ((uplim (+ (A000523 n) (- 1 (modulo (A000523 n) 2))))) (+ (* (expt 2 (+ 1 uplim)) n) (add (lambda (i) (* (bit_i n (+ i (expt -1 i))) (expt 2 (- uplim i)))) 0 uplim))))) (define (bit_i n i) (modulo (floor->exact (/ n (expt 2 i))) 2)) ;; The functional add implements sum_{i=lowlim..uplim} intfun(i): (define (add intfun lowlim uplim) (let sumloop ((i lowlim) (res 0)) (cond ((> i uplim) res) (else (sumloop (1+ i) (+ res (intfun i))))))) ;; Another version based on using A030103: (define (A048703v2 n) (if (zero? n) n (+ (* (A000302 (A110591 n)) n) (A030103 n)))) (Python) def A048703(n):     s = bin(n-1)[2:]     if len(s) % 2: s = '0'+s     t = [s[i:i+2] for i in range(0, len(s), 2)]     return int(''.join(t+t[::-1]), 2) # Chai Wah Wu, Feb 26 2021 CROSSREFS Subsequence of A014192 (all numbers which are palindromes in base 4, including also those of odd number of digits). Cf. also A048704 (this sequence divided by 5), A048701 (binary palindromes of even length), A055948, A110591, A118595, A030103, A007090, A000302. Sequence in context: A022600 A067237 A196157 * A020332 A244025 A044830 Adjacent sequences:  A048700 A048701 A048702 * A048704 A048705 A048706 KEYWORD nonn,base AUTHOR Antti Karttunen, Mar 07 1999 STATUS approved

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Last modified September 24 01:27 EDT 2021. Contains 347617 sequences. (Running on oeis4.)