Original entry on oeis.org
1, 106, 11516, 1243426, 9688988696, 96978050779606, 971598777778061426, 9725080505141505352336, 101523541416869687141725106, 1017966959632335053323596697836
Offset: 1
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
2, 222, 24042, 2587862, 19460805582, 194783456187402, 1949652919092569222, 19514712827464728951042, 195328964640102010365332862, 2036926191873749194737819896582, 20387527373464782858287465275278402, 204057110181920737691967380200911660222, 2042389465473836468010601076554746548042042
Offset: 1
A078279
a(n) is the number from which if odd-positioned digits are deleted then one gets the concatenation of first n composite numbers, while if even-positioned digits are deleted then one gets reverse concatenation of first n composite numbers. Position of least significant digit is considered to be 1.
Original entry on oeis.org
44, 4664, 486684, 49688694, 416089981604, 4162819019081624, 41648192110019281644, 416581941102112019481654, 4166819511041122114019581664, 41688196110511241142115019681684
Offset: 1
a(4) = 49688694: deleting alternate digits starting from the LSD leaves 4689. Deleting the other ones leaves 9864.
More terms from Antonio G. Astudillo (afg_astudillo(AT)lycos.com), Apr 19 2003
Original entry on oeis.org
2, 212, 22122, 2258577, 18913180982, 189219046321892, 1893099641364512802, 18935542777368728158257, 189400886865505550637253712, 1894918005023238233732500894622, 19367362777550568701870550728176482, 193719500503687550659615507369001321937, 1937649596364149143005250529144645910417392
Offset: 1
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a(n) = {my(c=vector(n), d=0, v=w=[4]); forcomposite(i=4, 4*n, c[d++]=i; if(d==n, break)); for(i=2, n, v=concat(v, d=digits(c[i])); w=concat(d, w)); fromdigits(vector(2*#v, i, if(i%2, v[1+i\2], w[i/2])))/22; } \\ Jinyuan Wang, May 17 2020
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