A394408 Lesser of two consecutive numbers of the form 2^i*5^j with i, j >= 0, that are coprime.

1, 4, 5, 25, 125, 512, 65536, 8388608, 1073741824, 137438953472, 17592186044416, 2251799813685248, 288230376151711744, 36893488147419103232, 4656612873077392578125, 173472347597680709441192448139190673828125, 6427752177035961102167848369364650410088811975131171341205504

Offset

1,2

Links

Michael De Vlieger, Table of n, a(n) for n = 1..23

Michael De Vlieger, Extended table of n, s(i), a(n), b(n) for n = 1..29, where b = A394452 and s = A003592.

Example

Let s = A003592 and b = A394452.
Table of n, a(n) for n = 1..12:
 n          a(n)     b(n)
----------------------------
 1     s(1) = 1        2
 2     s(3) = 2^2      5
 3     s(4) = 5        2^3
 4     s(9) = 5^2      2^5
 5    s(16) = 5^3      2^7
 6    s(25) = 2^9      5^4
 7    s(68) = 2^16     5^7
 8   s(132) = 2^23     5^10
 9   s(217) = 2^30     5^13
10   s(323) = 2^37     5^16
11   s(450) = 2^44     5^19
12   s(598) = 2^51     5^22

Maple

M:= 10^80: # for terms <= M
L:= sort([seq(seq(2^i*5^j, i=0..floor(log[2](M/5^j))), j=0..floor(log[5](M)))]):
L[select(t -> igcd(L[t], L[t+1])=1, [$1..nops(L)-1])]; # Robert Israel, Mar 23 2026

Mathematica

s = With[{nn = 10^80}, Union@ Flatten@ Table[2^i* 5^j, {i, 0, Log2[nn]}, {j, 0, Log[5, nn/2^i] } ] ]; Select[Partition[s, 2, 1], CoprimeQ @@ # &][[;; , 1]]

Crossrefs

Cf. A003592, A186927, A393164, A394452.

Sequence in context: A047168 A220538 A187996 * A144053 A084465 A056200

Adjacent sequences: A394405 A394406 A394407 * A394409 A394410 A394411

Keyword

nonn

Author

Michael De Vlieger, Mar 20 2026

Status

approved