A078274
a(n) is the number from which if odd-positioned digits are deleted then one gets the concatenation of first n odd numbers, while if even-positioned digits are deleted then one gets reverse concatenation of first n odd numbers. Position of least significant digit is considered to be 1.
Original entry on oeis.org
11, 1331, 153351, 17355371, 1937557391, 11315977951311, 113351719917151331, 1135517391111917351351, 11375175911311113917551371, 113951779115111331115917751391, 1231517991171115311351117917952311, 12335271911911173115511371119927152331
Offset: 1
a(4) = 17355371: deleting alternate digits starting from the LSD gives 1357. Deleting the other digits gives 7531.
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a(n) = {my(d, v=w=[1]); for(i=2, n, v=concat(v, d=digits(2*i-1)); w=concat(d, w)); fromdigits(vector(2*#v, i, if(i%2, v[1+i\2], w[i/2]))); } \\ Sean A. Irvine, Jun 26 2025
More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 19 2003
A078275
a(n) is the number from which if odd-positioned digits are deleted then one gets the concatenation of first n even numbers, while if even-positioned digits are deleted then one gets reverse concatenation of first n even numbers. Position of least significant digit is considered to be 1.
Original entry on oeis.org
22, 2442, 264462, 28466482, 214068861402, 2142618018061422, 21446182110018261442, 214661841102112018461462, 2148618611041122114018661482, 22406188110611241142116018862402, 224262801108112611441162118028062422, 2244628212001128114611641182210028262442
Offset: 1
a(4) = 28466482: deleting alternate digits starting from the LSD gives 2468. Deleting the other ones gives 8642.
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a(n) = {my(d, v=w=[2]); for(i=2, n, v=concat(v, d=digits(2*i)); w=concat(d, w)); fromdigits(vector(2*#v, i, if(i%2, v[1+i\2], w[i/2]))); } \\ Jinyuan Wang, May 17 2020
More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 19 2003
Original entry on oeis.org
1, 121, 13941, 1577761, 176141581, 1028725268301, 10304701810650121, 103228853737447031941, 1034106901028283083413761, 10359252646828302828719795581, 111956181015555937395556174359301, 1121388355628288465046488283629741121
Offset: 1
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a(n) = {my(d, v=w=[1]); for(i=2, n, v=concat(v, d=digits(2*i-1)); w=concat(d, w)); fromdigits(vector(2*#v, i, if(i%2, v[1+i\2], w[i/2])))/11; } \\ Jinyuan Wang, May 17 2020 [Updated by Sean A. Irvine, Jun 26 2025]
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